Fixed chrome selection bug (fixes #2151)

It seems that sometimes Chrome triggers a mousemove event before a click event with webkitMovementX == 0 and webkitMovementY == 0, which causes the behavior described in #2151.

Couldn't find a way to reproduce it on demand, but ignoring these events fixed it.

I've also found a few open bug entries regarding these phantom events in Chromium bug tracker.