Schemes for doing buffer readback:

1.) Copy to host and back:
	glReadPixels into buffer
	glTexSubImage2D to copy back to texture  
           glReadPixels(RGBA):  13.808 us + 716.679 ns/pixel
	   glTexImage2D(RGBA):  1.074 us + 4.024 ns/pixel

2.) Copy on graphics card
	glCopyTexSubImage2D from framebuffer to texture
           glCopyTexSubImage2D:  8.504 us + 1.656 ns/pixel

3.)