Since SDL2 is using in-memory bitmaps the same as Canvas engine, the
function names of the render.Engine interface have been cleaned up:
* NewTexture(filename, image) -> StoreTexture(name, image)
Create a new cached texture with a given name.
* NewBitmap(filename) -> LoadTexture(name)
Recall a stored texture with a given name.
* level.Chunk.ToBitmap uses simpler names for the textures instead of
userdir.CacheFilename file-like paths.
