brushlib: fix symmetry mode eating horiz strokes
brushlib: fix symmetry mode eating horiz strokes

Fix symmetry mode eating some mirrored brushstrokes under very specific
circumstances: https://gna.org/bugs/?20730

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