Sprite refactoring; rendering refactoring and toolset conversion

[lscharen]

This is a very large PR that implemented the following major changes

• rework sprite subsystem to eliminate the need to draw/erase sprites into a temporary buffer
• streamline the handling of tile dispatch and unify the dispatch paths for different engine modes
• specialize the rendering for different engine modes
• convert most of GTE into a user toolset; the tools and sprites demos have been converted to the toolset