* Add a Red Azulian as a test for mobile enemies.
* Its A.I. has it walk back and forth, changing directions when it
comes up against an obstacle for a few moments.
* It plays walking animations and can trigger collision events with
other Doodads, such as the Electric Door and Trapdoor.
* Move Gravity responsibility to the doodad scripts themselves.
* Call `Self.SetGravity(true)` to opt the Doodad in to gravity.
* The canvas.Loop() adds gravity to any doodad that has it enabled.
* Add animation support for Doodad actors (Play Mode) into the core
engine, so that the Doodad script can register named animations and
play them without managing all the details themselves.
* Doodad API functions on Self: AddAnimation, PlayAnimation,
StopAnimation, IsAnimating
* CLI: the `doodad convert` command will name each layer after the
filename used as the input image.
* CLI: fix the `doodad convert` command creating duplicate Palette
colors when converting a series of input images into a Doodad.
