Automatically load textures with higher resolution than the screen

Currently, we only automatically fall back to textures with a *lower* resolution than the screen that the game is running on (for example, if the game is running on an iPhone X and there's no `@3x` asset, then the `@2x` or `@1x` version will be automatically loaded).

We should do the same thing for falling back to a *higher* resolution as well, that is if the screen is an `@1x` screen but there's only an `@2x` asset, that asset should be loaded.

Texture loading takes place in `TextureManager` and `BundleTextureImageLoader`.