GL_ARB_texture_gather
http://www.opengl.org/registry/specs/ARB/texture_gather.txt
GL_ARB_texture_gather
	GL_MIN_PROGRAM_TEXTURE_GATHER_OFFSET_ARB 0x8E5E
	GL_MAX_PROGRAM_TEXTURE_GATHER_OFFSET_ARB 0x8E5F
	GL_MAX_PROGRAM_TEXTURE_GATHER_COMPONENTS_ARB 0x8F9F
