GL_SGIX_depth_pass_instrument
http://www.opengl.org/registry/specs/SGIX/depth_pass_instrument.txt
GL_SGIX_depth_pass_instrument

	GL_DEPTH_PASS_INSTRUMENT_SGIX 0x8310
	GL_DEPTH_PASS_INSTRUMENT_COUNTERS_SGIX 0x8311
	GL_DEPTH_PASS_INSTRUMENT_MAX_SGIX 0x8312