st/mesa: provide GL_OES_copy_image support by caching the original ETC data

st/mesa: provide GL_OES_copy_image support by caching the original ETC data

The additional provision of GL_OES_copy_image is that it work for ETC.
However many desktop GPUs don't have native ETC support, so st/mesa does
the decoding by hand. Instead of discarding the compressed data, keep it
around in CPU memory. Use it when performing image copies.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Acked-by: Marek Olšák <marek.olsak@amd.com>