st/mesa: add fake ARB_copy_image support in Gallium

st/mesa: add fake ARB_copy_image support in Gallium

This support should be removed in favor of something that actually works
in all the weird cases. However this is simple and is enough to allow
Bioshock Infinite to render properly on nvc0.

Since the functionality is not implemented correctly, the extension will
not appear in the extension string and mesa will still return
INVALID_OPERATION for any glCopyImageSubData calls. In order to make use
of this functionality, run with
MESA_EXTENSION_OVERRIDE=GL_ARB_copy_image

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