main: Nameless texture creation and deletion. Does not affect normal creation and...

main: Nameless texture creation and deletion. Does not affect normal creation and deletion paths.

In implementing ARB_DIRECT_STATE_ACCESS functions, it is often necessary to
abstract the functionality of a traditional GL API function into a backend
that both the traditional and dsa API functions can share.  For instance,
glTexParameteri and glTextureParameteri both call _mesa_texture_parameteri,
which takes a context object and a texture object as arguments.

The existance of such backend functions provides the opportunity for
driver internals (such as meta) to pass around the actual texture object
rather than its ID or target, saving on texture object storage and look-up
overhead.

This patch provides nameless texture creation and deletion for meta.  This
will be used in an upcoming refactor of meta.

Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>