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.
Sheets carry a managed buffer of data that allows updates to be made without having to constantly fetch and set data to the texture memory of the video card. This is useful for things like SheetBuilder which make small progressive changes to sheets.
However these buffers are often large and are kept alive because sheets are referenced by the sprites that use them. If this buffer is explicitly null'ed when it is no longer needed then the GC can reclaim it. Sometimes a buffer need not even be created because the object using the sheet only works on the texture directly anyway.
In practise, this reduced memory consumed by such buffers from ~165 MiB to ~112 MiB (at the start of a new RA skirmish mission).
