GL_SGIX_depth_texture
http://oss.sgi.com/projects/ogl-sample/registry/SGIX/depth_texture.txt
	GL_DEPTH_COMPONENT16_SGIX 0x81A5
	GL_DEPTH_COMPONENT24_SGIX 0x81A6
	GL_DEPTH_COMPONENT32_SGIX 0x81A7
