brushlib: add reset, reset at startup
brushlib: add reset, reset at startup

Avoid interpolating a stroke from (0, 0) if the first event that is
fed into the brush engine already has pressure. This removes the
empty borders from the PNGs generated by test_mypaintlib.py.

Note: this could affect some of the performance tests.

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