Textures, FrameBuffers and VertexBuffers allocated by the Sdl2 Renderer were only being released via finalizers. This could lead to OpenGL out of memory errors since resources may not be cleaned up in a timely manner. To avoid this, IDisposable has been implemented and transitively applied to classes that use these resources.
As a side-effect some static state is no longer static, particularly in Renderer, in order to facilitate this change and just for nicer design in general.
Also dispose some bitmaps.
The managed byte buffer is created on demand, meaning a newly allocated sheet will not waste memory holding onto the buffer until some changes are actually required to be written. This avoids a newly allocated sheet wasting memory on buffers that do not differ from their backing texture.
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).
