GL_ARB_depth_texture
http://www.opengl.org/registry/specs/ARB/depth_texture.txt
	GL_DEPTH_COMPONENT16_ARB 0x81A5
	GL_DEPTH_COMPONENT24_ARB 0x81A6
	GL_DEPTH_COMPONENT32_ARB 0x81A7
	GL_TEXTURE_DEPTH_SIZE_ARB 0x884A
	GL_DEPTH_TEXTURE_MODE_ARB 0x884B
