brushlib: Mark functions in tiled-surface where thread-safety applies
brushlib: Mark functions in tiled-surface where thread-safety applies

Also unneeded MyPaintTiledSurface argument to process_op.

b3sigma/libmypaint,achadwick/libmypaint,b3sigma/libmypaint,achadwick/libmypaint,achadwick/libmypaint,achadwick/libmypaint,b3sigma/libmypaint