st/mesa: implement AllocTextureImageBuffer() driver hook

st/mesa: implement AllocTextureImageBuffer() driver hook

This hasn't been needed so far since none of the core Mesa code paths
that call ctx->Driver.AllocTextureImageBuffer() are used with the
state tracker.  That will change in upcoming patches.
Note that this function duplicates some code seen in the st_TexImage()
function.  That can be cleaned up later.