* Add sync.WaitGroup to some parts of the level collision detection
function and Canvas.Loop() to speed up the frame rate by load
balancing some work in parallel across multiple cores.
* Improves FPS from 30 to 55+ even for busy scenes with lots of mobile
enemies walking around.
* Before the level collision optimization, framerate would sometimes dip
to 30 FPS simply to move the player character on a completely blank
map!
* 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.
Noah