st/mesa: implement a simple cache for glDrawPixels

st/mesa: implement a simple cache for glDrawPixels

Instead of discarding the texture we created, keep it around in case
the next glDrawPixels draws the same image again.  This is intended
to help application which draw the same image several times in a row,
either within a frame or subsequent frames.

Reviewed-by: Charmaine Lee <charmainel@vmware.com>