st/mesa: fix memleak in glDrawPixels cache code

st/mesa: fix memleak in glDrawPixels cache code

If the glDrawPixels size changed, we leaked the previously cached
texture, if there was one.  This patch fixes the reference counting,
adds a refcount assertion check, and better handles potential malloc()
failures.

Tested with a modified version of the drawpix Mesa demo which changed
the image size for each glDrawPixels call.

Cc: "11.2" <mesa-stable@lists.freedesktop.org>
Reviewed-by: José Fonseca <jfonseca@vmware.com>
Reviewed-by: Charmaine Lee <charmainel@vmware.com>