glFeedbackBuffer - controls feedback mode

	  void glFeedbackBuffer( GLsizei size,
				 GLenum	type,
				 GLfloat *buffer )

	  size	  Specifies the	maximum	number of values that can be
		  written into buffer.

	  type	  Specifies a symbolic constant	that describes the
		  information that will	be returned for	each vertex.
		  GL_4D_COLOR_TEXTURE are accepted.

	  buffer  Returns the feedback data.

	  The glFeedbackBuffer function	controls feedback.  Feedback,
	  like selection, is a GL mode.	 The mode is selected by
	  calling glRenderMode with GL_FEEDBACK.  When the GL is in
	  feedback mode, no pixels are produced	by rasterization.
	  Instead, information about primitives	that would have	been
	  rasterized is	fed back to the	application using the GL.

	  glFeedbackBuffer has three arguments:	 buffer	is a pointer
	  to an	array of floating-point	values into which feedback
	  information is placed.  size indicates the size of the
	  array.  type is a symbolic constant describing the
	  information that is fed back for each	vertex.
	  glFeedbackBuffer must	be issued before feedback mode is
	  enabled (by calling glRenderMode with	argument GL_FEEDBACK).
	  Setting GL_FEEDBACK without establishing the feedback
	  buffer, or calling glFeedbackBuffer while the	GL is in
	  feedback mode, is an error.

	  When glRenderMode is called while in feedback	mode, it
	  returns the number of	entries	placed in the feedback array,
	  and resets the feedback array	pointer	to the base of the
	  feedback buffer. The returned	value never exceeds size. If
	  the feedback data required more room than was	available in
	  buffer, glRenderMode returns a negative value.  To take the
	  GL out of feedback mode, call	glRenderMode with a parameter
	  value	other than GL_FEEDBACK.

	  While	in feedback mode, each primitive, bitmap, or pixel
	  rectangle that would be rasterized generates a block of
	  values that are copied into the feedback array.  If doing so
	  would	cause the number of entries to exceed the maximum, the
	  block	is partially written so	as to fill the array (if there
	  is any room left at all), and	an overflow flag is set.  Each
	  block	begins with a code indicating the primitive type,
	  followed by values that describe the primitive's vertices
	  and associated data.	Entries	are also written for bitmaps
	  and pixel rectangles.	 Feedback occurs after polygon culling
	  and glPolygonMode interpretation of polygons has taken
	  place, so polygons that are culled are not returned in the
	  feedback buffer.  It can also	occur after polygons with more
	  than three edges are broken up into triangles, if the	GL
	  implementation renders polygons by performing	this

	  The glPassThrough command can	be used	to insert a marker
	  into the feedback buffer.  See glPassThrough.

	  Following is the grammar for the blocks of values written
	  into the feedback buffer.  Each primitive is indicated with
	  a unique identifying value followed by some number of
	  vertices.  Polygon entries include an	integer	value
	  indicating how many vertices follow.	A vertex is fed	back
	  as some number of floating-point values, as determined by
	  type.	 Colors	are fed	back as	four values in RGBA mode and
	  one value in color index mode.

	       feedbackList <- feedbackItem feedbackList |

	       feedbackItem <- point | lineSegment | polygon | bitmap
	       | pixelRectangle	| passThru

	       point <-	GL_POINT_TOKEN vertex

	       lineSegment <- GL_LINE_TOKEN vertex vertex |
	       GL_LINE_RESET_TOKEN vertex vertex

	       polygon <- GL_POLYGON_TOKEN n polySpec

	       polySpec	<- polySpec vertex | vertex vertex vertex

	       bitmap <- GL_BITMAP_TOKEN vertex

	       pixelRectangle <- GL_DRAW_PIXEL_TOKEN vertex |
	       GL_COPY_PIXEL_TOKEN vertex

	       passThru	<- GL_PASS_THROUGH_TOKEN value

	       vertex <- 2d | 3d | 3dColor | 3dColorTexture |

	       2d <- value value
	       3d <- value value value

	       3dColor	<- value value value color

	       3dColorTexture <- value value value color tex

	       4dColorTexture <- value value value value color tex

	       color <-	rgba | index

	       rgba <- value value value value

	       index <-	value

	       tex <- value value value	value

	  value	is a floating-point number, and	n is a floating-point
	  integer giving the number of vertices	in the polygon.
	  floating-point constants.  GL_LINE_RESET_TOKEN is returned
	  whenever the line stipple pattern is reset.  The data
	  returned as a	vertex depends on the feedback type.

	  The following	table gives the	correspondence between type
	  and the number of values per vertex.	k is 1 in color	index
	  mode and 4 in	RGBA mode.

     |	     type	  | coordinates	| color	| texture | total number of values |
     |	     GL_2D	  |    x, y	|	|	  |	      2		   |
     |	     GL_3D	  |   x, y, z	|	|	  |	      3		   |
     |	  GL_3D_COLOR	  |   x, y, z	|   k	|	  |	     3+k	   |
     |GL_3D_COLOR_TEXTURE |  x,	y, z,	|   k	|    4	  |	     7+k	   |
     |GL_4D_COLOR_TEXTURE | x, y, z, w	|   k	|    4	  |	     8+k	   |

	  Feedback vertex coordinates are in window coordinates,
	  except w, which is in	clip coordinates.  Feedback colors are
	  lighted, if lighting is enabled.  Feedback texture
	  coordinates are generated, if	texture	coordinate generation
	  is enabled.  They are	always transformed by the texture

	  glFeedbackBuffer, when used in a display list, is not
	  compiled into	the display list but is	executed immediately.

	  GL_INVALID_ENUM is generated if type is not an accepted

	  GL_INVALID_VALUE is generated	if size	is negative.

	  GL_INVALID_OPERATION is generated if glFeedbackBuffer	is
	  called while the render mode is GL_FEEDBACK, or if
	  glRenderMode is called with argument GL_FEEDBACK before
	  glFeedbackBuffer is called at	least once.

	  GL_INVALID_OPERATION is generated if glFeedbackBuffer	is
	  executed between the execution of glBegin and	the
	  corresponding	execution of glEnd.

	  glGet	with argument GL_RENDER_MODE

	  glBegin, glLineStipple, glPassThrough, glPolygonMode,
	  glRenderMode,	glSelectBuffer