st/mesa: add destroy_drawable interface

st/mesa: add destroy_drawable interface

With this patch, the st manager will maintain a hash table for
the active framebuffer interface objects. A destroy_drawable interface
is added to allow the state tracker to notify the st manager to remove
the associated framebuffer interface object from the hash table,
so the associated framebuffer and its resources can be deleted
at framebuffers purge time.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101829
Fixes: 147d7fb772a ("st/mesa: add a winsys buffers list in st_context")
Tested-by: Brad King <brad.king@kitware.com>
Tested-by: Gert Wollny <gw.fossdev@gmail.com>
Reviewed-by: Brian Paul <brianp@vmware.com>