GL_SGIX_depth_texture
https://www.khronos.org/registry/OpenGL/extensions/SGIX/SGIX_depth_texture.txt
GL_SGIX_depth_texture

	GL_DEPTH_COMPONENT16_SGIX 0x81A5
	GL_DEPTH_COMPONENT24_SGIX 0x81A6
	GL_DEPTH_COMPONENT32_SGIX 0x81A7