* 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
