st/mesa: expand glDrawPixels cache to handle multiple images

st/mesa: expand glDrawPixels cache to handle multiple images

The newest version of WSI Fusion makes several glDrawPixels calls
per frame.  By caching more than one image, we get better performance
when panning/zooming the map.

v2: move pixel unpack param checking out of cache search loop, per Roland
v3: also move unpack->BufferObj check out of loop, per Roland.