Add multi-touch gesture support so that the player can scroll the level
in the editor (and title screen) by treating a two finger swipe to be
equivalent to a middle click drag.
Fun quirks found with SDL2's MultiGestureEvent:
* They don't begin sending us the event until motion is detected after
two fingers have touched the screen; not the moment the second finger
touches it.
* It spams us with events when it detects any tiny change and a lot of
cool details like rotate/pinch deltas, but it never tells us when the
multitouch STOPS! The game has to block left clicks while multitouch
happens so the user doesn't draw all over their level, so it needs to
know when touch has ended.
* The workaround is to track the mouse cursor position at the first
touch and each delta thereafter; if the deltas stop changing tick to
tick, unset the "is touching" variable.
Noah Petherbridge
a24c94a161