svga: Support ARB_buffer_storage

svga: Support ARB_buffer_storage

This basically boils down to supporting persistent and coherent buffer
storage.
We chose to use coherent buffer storage for all persistent buffers
even if it's not explicitly specified, since using glMemoryBarrier to
obtain coherency would be particularly expensive in our driver stack,
and require a lot of additional bookkeeping.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>