Fix bug causing incorrect movement values in mouse events

This adds code to correctly set the movementX and movementY members of EmscriptenMouseEvents sent to callback functions by the HTML5 event library. movementX and movementY are not members of the base HTML5 mouse event, so we need to add extra code to retrieve them for our events. Without this commit, movementX and movementY are always 0, which makes capturing the pointer useless.