Preprocess texture assets in editor to improve loading speed at runtime

[aleksigron]

We need to be able to preprocess texture assets in the editor and store the results in some kind of cache folder. Then those results are used when loading textures at runtime.

We need to be able specify

• Preprocess in the editor or load from source image at runtime
• Size used at runtime

Depends on #81. The options added from that issue should also respect the setting whether the processing happens in editor or at runtime.