glScissor - define the scissor box

	  void glScissor( GLint	x,
			  GLint	y,
			  GLsizei width,
			  GLsizei height )

	  x, y Specify the lower left corner of	the scissor box.
	       Initially (0, 0).

	  width, height
	       Specify the width and height of the scissor box.	 When
	       a GL context is first attached to a window, width and
	       height are set to the dimensions	of that	window.

	  glScissor defines a rectangle, called	the scissor box, in
	  window coordinates.  The first two arguments,	x and y,
	  specify the lower left corner	of the box.  width and height
	  specify the width and	height of the box.

	  To enable and	disable	the scissor test, call glEnable	and
	  glDisable with argument GL_SCISSOR_TEST. The test is
	  initially disabled. While the	test is	enabled, only pixels
	  that lie within the scissor box can be modified by drawing
	  commands.  Window coordinates	have integer values at the
	  shared corners of frame buffer pixels.  glScissor(0,0,1,1)
	  allows modification of only the lower	left pixel in the
	  window, and glScissor(0,0,0,0) doesn't allow modification of
	  any pixels in	the window.

	  When the scissor test	is disabled, it	is as though the
	  scissor box includes the entire window.

	  GL_INVALID_VALUE is generated	if either width	or height is

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

	  glGet	with argument GL_SCISSOR_BOX
	  glIsEnabled with argument GL_SCISSOR_TEST


	  glEnable, glViewport