GL_IMG_framebuffer_downsample
https://www.khronos.org/registry/OpenGL/extensions/IMG/IMG_framebuffer_downsample.txt
GL_IMG_framebuffer_downsample

	GL_FRAMEBUFFER_INCOMPLETE_MULTISAMPLE_AND_DOWNSAMPLE_IMG 0x913C
	GL_NUM_DOWNSAMPLE_SCALES_IMG 0x913D
	GL_DOWNSAMPLE_SCALES_IMG 0x913E
	GL_FRAMEBUFFER_ATTACHMENT_TEXTURE_SCALE_IMG 0x913F
	void glFramebufferTexture2DDownsampleIMG (GLenum target, GLenum attachment, GLenum textarget, GLuint texture, GLint level, GLint xscale, GLint yscale)
	void glFramebufferTextureLayerDownsampleIMG (GLenum target, GLenum attachment, GLuint texture, GLint level, GLint layer, GLint xscale, GLint yscale)