glStencilOp -	set stencil test actions

	  void glStencilOp( GLenum fail,
			    GLenum zfail,
			    GLenum zpass )

	  fail	 Specifies the action to take when the stencil test
		 fails.	 Six symbolic constants	are accepted:
		 GL_INVERT. The	initial	value is GL_KEEP.

	  zfail	 Specifies the stencil action when the stencil test
		 passes, but the depth test fails.  zfail accepts the
		 same symbolic constants as fail. The initial value is

	  zpass	 Specifies the stencil action when both	the stencil
		 test and the depth test pass, or when the stencil
		 test passes and either	there is no depth buffer or
		 depth testing is not enabled.	zpass accepts the same
		 symbolic constants as fail. The initial value is

	  Stenciling, like depth-buffering, enables and	disables
	  drawing on a per-pixel basis.	 You draw into the stencil
	  planes using GL drawing primitives, then render geometry and
	  images, using	the stencil planes to mask out portions	of the
	  screen.  Stenciling is typically used	in multipass rendering
	  algorithms to	achieve	special	effects, such as decals,
	  outlining, and constructive solid geometry rendering.

	  The stencil test conditionally eliminates a pixel based on
	  the outcome of a comparison between the value	in the stencil
	  buffer and a reference value.	To enable and disable the
	  test,	call glEnable and glDisable with argument
	  GL_STENCIL_TEST; to control it, call glStencilFunc.

	  glStencilOp takes three arguments that indicate what happens
	  to the stored	stencil	value while stenciling is enabled.  If
	  the stencil test fails, no change is made to the pixel's
	  color	or depth buffers, and fail specifies what happens to
	  the stencil buffer contents.	The following six actions are

	  GL_KEEP	  Keeps	the current value.

	  GL_ZERO	  Sets the stencil buffer value	to 0.

	  GL_REPLACE	  Sets the stencil buffer value	to ref,	as
			  specified by glStencilFunc.

	  GL_INCR	  Increments the current stencil buffer	value.
			  Clamps to the	maximum	representable unsigned

	  GL_DECR	  Decrements the current stencil buffer	value.
			  Clamps to 0.

	  GL_INVERT	  Bitwise inverts the current stencil buffer

	  Stencil buffer values	are treated as unsigned	integers.
	  When incremented and decremented, values are clamped to 0
	  and 2n-1, where n is the value returned by querying

	  The other two	arguments to glStencilOp specify stencil
	  buffer actions that depend on	whether	subsequent depth
	  buffer tests succeed (zpass) or fail (zfail) (see
	  glDepthFunc).	 The actions are specified using the same six
	  symbolic constants as	fail.  Note that zfail is ignored when
	  there	is no depth buffer, or when the	depth buffer is	not
	  enabled.  In these cases, fail and zpass specify stencil
	  action when the stencil test fails and passes, respectively.

	  Initially the	stencil	test is	disabled.  If there is no
	  stencil buffer, no stencil modification can occur and	it is
	  as if	the stencil tests always pass, regardless of any call
	  to glStencilOp.

	  GL_INVALID_ENUM is generated if fail,	zfail, or zpass	is any
	  value	other than the six defined constant values.

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

	  glGet	with argument GL_STENCIL_FAIL
	  glGet	with argument GL_STENCIL_PASS_DEPTH_PASS
	  glGet	with argument GL_STENCIL_PASS_DEPTH_FAIL
	  glGet	with argument GL_STENCIL_BITS
	  glIsEnabled with argument GL_STENCIL_TEST

	  glAlphaFunc, glBlendFunc, glDepthFunc, glEnable, glLogicOp,