Due to Windows doesn't support dnd in the pen message handler, we can't handle and consume WM_POINTER* to fire WidgetMouseEvent. On the other hand, we can't get some pen related attributes from Windows mouse messages. This patch gets and caches the attributes by handling WM_POINTER* but not fire WidgetMouseEvent to support dnd and pen related attributes. When handling the subsequent Windows mouse messages, we use the cached attributes to fire WidgetMouseEvent. Considering we might need to use WM_POINTER* someday, use a preference to control the behavior.
MozReview-Commit-ID: 5E60KO1zo0W
