GL_SGIX_async_pixel
http://www.opengl.org/registry/specs/SGIX/async_pixel.txt
	GL_ASYNC_TEX_IMAGE_SGIX 0x835C
	GL_ASYNC_DRAW_PIXELS_SGIX 0x835D
	GL_ASYNC_READ_PIXELS_SGIX 0x835E
	GL_MAX_ASYNC_TEX_IMAGE_SGIX 0x835F
	GL_MAX_ASYNC_DRAW_PIXELS_SGIX 0x8360
	GL_MAX_ASYNC_READ_PIXELS_SGIX 0x8361
