GL_SGIX_depth_texture
http://www.opengl.org/registry/specs/gl/SGIX/depth_texture.txt
GL_SGIX_depth_texture
	GL_DEPTH_COMPONENT16_SGIX 0x81A5
	GL_DEPTH_COMPONENT24_SGIX 0x81A6
	GL_DEPTH_COMPONENT32_SGIX 0x81A7