* 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.
* pkg/loadscreen implements a global Loading Screen for loading heavy
levels for playing or editing.
* All chunks in a level are pre-rendered to bitmap before gameplay
begins, which reduces stutter as chunks were being lazily rendered on
first appearance before.
* The loading screen can be played with in the developer console:
$ loadscreen.Show()
$ loadscreen.Hide()
Along with ShowWithProgress(), SetProgress(float64) and IsActive()
* Chunker: separate the concerns between Bitmaps an (SDL2) Textures.
* Chunker.Prerender() converts a chunk to a bitmap (a Go image.Image)
and caches it, only re-rendering if marked as dirty.
* Chunker.Texture() will use the pre-cached bitmap if available to
immediately produce the SDL2 texture.
Other miscellaneous changes:
* Added to the Colored Pencil palette: Sandstone
* Added "perlin noise" brush pattern
Note: this commit introduces instability and crashes:
* New `asyncSetup()` functions run on a goroutine, but SDL2 texture
calls must run on the main thread.
* Chunker avoids this by caching bitmaps, not textures.
* Wallpaper though is unstable, sometimes works, sometimes has graphical
glitches, sometimes crashes the game.
* Wallpaper.Load() and the *Texture() functions are where it crashes.
