brushlib: clear bbox in m.t.s._begin_atomic()
brushlib: clear bbox in m.t.s._begin_atomic()

Fix unnecessarily large and frequent screen redraws in MyPaint:
dirty_bbox should not accumulate across multiple brushstrokes.

https://gna.org/bugs/?20371

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