mesa/st: implement memory objects as a backend for texture storage

mesa/st: implement memory objects as a backend for texture storage

Instead of allocating memory to back a texture, use the provided memory
object.

v2: split off extension exposure logic
v3: de-duplicate code with st_AllocTextureStorage

Signed-off-by: Andres Rodriguez <andresx7@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>