The old way was to use big switch/case statements based on a type of buffer.
The new one is to use inheritance.
This change prohibits us to change the buffer type while running, but I doubt we'll ever do so.
Performance should also be a bit better. Also a nice cleanup.
Added some comments about this different kind of buffers.
This is a bit slower on map_and_* because of flushing and _very_ much slower on buffer(sub)?data because of a new memcpy.
But this design allow us to decode directly into a gpu buffer, eg vertexloader will profit :)
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