glAccum - operate on the accumulation	buffer

	  void glAccum(	GLenum op,
			GLfloat	value )

	  op	 Specifies the accumulation buffer operation.
		 Symbolic constants GL_ACCUM, GL_LOAD, GL_ADD,
		 GL_MULT, and GL_RETURN	are accepted.

	  value	 Specifies a floating-point value used in the
		 accumulation buffer operation.	 op determines how
		 value is used.

	  The accumulation buffer is an	extended-range color buffer.
	  Images are not rendered into it.  Rather, images rendered
	  into one of the color	buffers	are added to the contents of
	  the accumulation buffer after	rendering.  Effects such as
	  antialiasing (of points, lines, and polygons), motion	blur,
	  and depth of field can be created by accumulating images
	  generated with different transformation matrices.

	  Each pixel in	the accumulation buffer	consists of red,
	  green, blue, and alpha values.  The number of	bits per
	  component in the accumulation	buffer depends on the
	  implementation. You can examine this number by calling
	  glGetIntegerv	four times, with arguments GL_ACCUM_RED_BITS,
	  GL_ACCUM_ALPHA_BITS.	Regardless of the number of bits per
	  component, the range of values stored	by each	component is
	  [-1, 1].  The	accumulation buffer pixels are mapped one-to-
	  one with frame buffer	pixels.

	  glAccum operates on the accumulation buffer.	The first
	  argument, op,	is a symbolic constant that selects an
	  accumulation buffer operation.  The second argument, value,
	  is a floating-point value to be used in that operation.
	  Five operations are specified:  GL_ACCUM, GL_LOAD, GL_ADD,

	  All accumulation buffer operations are limited to the	area
	  of the current scissor box and applied identically to	the
	  red, green, blue, and	alpha components of each pixel.	 If a
	  glAccum operation results in a value outside the range [-1,
	  1], the contents of an accumulation buffer pixel component
	  are undefined.

	  The operations are as	follows:

	  GL_ACCUM	Obtains	R, G, B, and A values from the buffer
			currently selected for reading (see
			glReadBuffer).	Each component value is
			divided	by 2n-1, where n is the	number of bits
			allocated to each color	component in the
			currently selected buffer.  The	result is a
			floating-point value in	the range [0, 1],
			which is multiplied by value and added to the
			corresponding pixel component in the
			accumulation buffer, thereby updating the
			accumulation buffer.

	  GL_LOAD	Similar	to GL_ACCUM, except that the current
			value in the accumulation buffer is not	used
			in the calculation of the new value.  That is,
			the R, G, B, and A values from the currently
			selected buffer	are divided by 2n-1,
			multiplied by value, and then stored in	the
			corresponding accumulation buffer cell,
			overwriting the	current	value.

	  GL_ADD	Adds value to each R, G, B, and	A in the
			accumulation buffer.

	  GL_MULT	Multiplies each	R, G, B, and A in the
			accumulation buffer by value and returns the
			scaled component to its	corresponding
			accumulation buffer location.

	  GL_RETURN	Transfers accumulation buffer values to	the
			color buffer or	buffers	currently selected for
			writing.  Each R, G, B,	and A component	is
			multiplied by value, then multiplied by	2n-1,
			clamped	to the range [0,2n-1], and stored in
			the corresponding display buffer cell.	The
			only fragment operations that are applied to
			this transfer are pixel	ownership, scissor,
			dithering, and color writemasks.

	  To clear the accumulation buffer, call glClearAccum with R,
	  G, B,	and A values to	set it to, then	call glClear with the
	  accumulation buffer enabled.

	  Only pixels within the current scissor box are updated by a
	  glAccum operation.

	  GL_INVALID_ENUM is generated if op is	not an accepted	value.

	  GL_INVALID_OPERATION is generated if there is	no
	  accumulation buffer.

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

	  glGet	with argument GL_ACCUM_RED_BITS
	  glGet	with argument GL_ACCUM_GREEN_BITS
	  glGet	with argument GL_ACCUM_BLUE_BITS
	  glGet	with argument GL_ACCUM_ALPHA_BITS

	  glBlendFunc, glClear,	glClearAccum, glCopyPixels, glGet,
	  glLogicOp, glPixelStore, glPixelTransfer, glReadBuffer,
	  glReadPixels,	glScissor, glStencilOp