* Move all collision code into the pkg/collision package.
* pkg/doodads/collision.go -> pkg/collision/collide_level.go
* pkg/doodads/collide_actors.go for new Actor collide support
* Add initial collision detection code between actors in Play Mode.
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.
