Passing the getUsedFrames argument will cause frames to be buffered
locally (and not added to the SheetBuilder) until they are explicitly
requested. This allows unused frames in sprite files to be skipped
instead of wasting space in GPU memory.
We split the caching SpriteLoader into a SpriteCache and FrameCache. SpriteLoader instead becomes a holder for static loading methods.
Only a few classes loaded sprite frames, and they all use it with a transient cache. By moving this method into a new class, we can lose the now redundant frame cache, saving on memory significantly since the frame data array can be reclaimed by the GC. This saves ~58 MiB on frames and ~4 MiB on the caching dictionary in simple tests.
