Play Mode: Fix Level Collision w/ Scrolling

Fixes:
* Move the call to CollidesWithGrid() inside the Canvas instead of
  outside in the PlayScene.movePlayer() so it can apply to all Actors
  in motion.
* PlayScene.movePlayer() in turn just sets the player's Velocity so the
  Canvas.Loop() can move the actor itself.
* When keeping the player inside the level boundaries: previously it was
  assuming the player Position was relative to the window, and was
  checking the WorldIndexAt and getting wrong results.
* Canvas scrolling (loopFollowActor): check that the actor is getting
  close to the screen edge using the Viewport into the world, NOT the
  screen-relative coordinates of the Canvas bounding boxes.

interface.go

@@ -135,6 +135,19 @@ func (r Rect) AddPoint(other Point) Rect {
 	}
 }
 
+// SubtractPoint is the inverse of AddPoint. Use this only if you need to invert
+// the Point being added.
+//
+// This does r.X - other.X, r.Y - other.Y and keeps the width/height the same.
+func (r Rect) SubtractPoint(other Point) Rect {
+	return Rect{
+		X: r.X - other.X,
+		Y: r.Y - other.Y,
+		W: r.W,
+		H: r.H,
+	}
+}
+
 // Text holds information for drawing text.
 type Text struct {
 	Text         string

point.go

@@ -75,6 +75,12 @@ func (p *Point) Add(other Point) {
 	p.Y += other.Y
 }
 
+// Subtract the other point from your current point.
+func (p *Point) Subtract(other Point) {
+	p.X -= other.X
+	p.Y -= other.Y
+}
+
 // MarshalText to convert the point into text so that a render.Point may be used
 // as a map key and serialized to JSON.
 func (p *Point) MarshalText() ([]byte, error) {