* The loading screen for Edit and Play modes is stable and the risk of
game crash is removed. The root cause was the setupAsync() functions
running on a background goroutine, and running SDL2 draw functions
while NOT on the main thread, which causes problems.
* The fix is all SDL2 Texture draws become lazy loaded: when the main
thread is presenting, any Wallpaper or ui.Image that has no texture
yet gets one created at that time from the cached image.Image.
* All internal game logic then uses image.Image types, to cache bitmaps
of Level Chunks, Wallpaper images, Sprite icons, etc. and the game is
free to prepare these asynchronously; only the main thread ever
Presents and the SDL2 textures initialize on first appearance.
* Several functions had arguments cleaned up: Canvas.LoadLevel() does
not need the render.Engine as (e.g. wallpaper) textures don't render
at that stage.
* Tightens up the surface area of API methods available to the
JavaScript VMs for doodads. Variables and functions are carefully
passed in one-by-one so the doodad script can only access intended
functions and not snoop on undocumented APIs.
* Wrote tons of user documentation for Doodad Scripts: documented the
full surface area of the exposed JavaScript API now that the surface
area is known and limited.
* Early WIP code for the Campaign JSON
