* 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!
* CLI: fix the `doodad convert` command to share the same Palette when
converting each frame (layer) of a doodad so subsequent layers find
the correct color swatches for serialization.
* Scripting: add timers and intervals to Doodad scripts to allow them to
animate themselves or add delayed callbacks. The timers have the same
API as a web browser: setTimeout(), setInterval(), clearTimeout(),
clearInterval().
* Add support for uix.Actor to change its currently rendered layer in
the level. For example a Button Doodad can set its image to Layer 1
(pressed) when touched by the player, and Trapdoors can cycle through
their layers to animate opening and closing.
* Usage from a Doodad script: Self.ShowLayer(1)
* Default Doodads: added scripts for all Buttons, Doors, Keys and the
Trapdoor to run their various animations when touched (in the case of
Keys, destroy themselves when touched, because there is no player
inventory yet)
* 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.
