Multitouch Level Panning

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.

pkg/uix/canvas.go

@@ -99,6 +99,7 @@ type Canvas struct {
 	scrollDragging  bool         // Middle-click to pan scroll
 	scrollStartAt   render.Point // Cursor point at beginning of pan
 	scrollWasAt     render.Point // copy of Scroll at beginning of pan
+	scrollLastDelta render.Point // multitouch spam
 }
 
 // NewCanvas initializes a Canvas widget.

pkg/uix/canvas_scrolling.go

@@ -54,8 +54,41 @@ func (w *Canvas) loopEditorScroll(ev *event.State) error {
 		w.ScrollBy(scrollBy)
 	}
 
+	// Multitouch events to pan the level, like middle click on desktop.
+	if ev.Touching {
+		// Intention: user drags with 2 fingers to scroll the canvas.
+		// SDL2 will register one finger also as a Button1 mouse click.
+		// We need to record the "mouse cursor" as start point but then
+		// fake that no click occurs so we don't nick the drawing.
+		if !w.scrollDragging {
+			w.scrollDragging = true
+			w.scrollStartAt = shmem.Cursor
+			w.scrollWasAt = w.Scroll
+			w.scrollLastDelta = render.Point{}
+		} else {
+			delta := shmem.Cursor.Compare(w.scrollStartAt)
+			w.Scroll = w.scrollWasAt
+			w.Scroll.Subtract(delta)
+
+			// So, SDL2 spams us with events for every subtle movement of 2+ fingers
+			// on the screen, but we don't know when that STOPS. As a heuristic, it
+			// seems we can tell by if the delta stops updating.
+			if !w.scrollLastDelta.IsZero() {
+				if w.scrollLastDelta == delta {
+					ev.Touching = false
+				}
+			}
+
+			w.scrollLastDelta = delta
+		}
+
+		// Lift the mouse button.
+		ev.Button1 = false
+
+		return nil
+	}
+
 	// Middle click of the mouse to pan the level.
-	// NOTE: returns are below!
 	if keybind.MiddleClick(ev) {
 		if !w.scrollDragging {
 			w.scrollDragging = true