... with several threads

Commit d8ae5817 didn't go far
enough in protecting GimpTileBackendPlugin against race conditions.
The underlying GimpTile cache is shared across all drawables, so we
must use a common lock for all instances of GimpTileBackendPlugin,
instead of one per instance.

Do just that -- replace the per-instance mutex of
GimpTileBackendPlugin with a global one.  This makes
GimpTileBackendPlugin instances thread-safe w.r.t. themselves, and
w.r.t other GimpTileBackendPlugin instances.  However, we don't aim
to make GimpTileBackendPlugin thread-safe w.r.t. other libgimp
functions at this point, since the original API has never been
thread-safe.