Bug fixes for World2D and entity classes - further progress on world demo and UI library
-Fixed world-to-screen/screen-to-world conversions not working correctly with PPU
-Changed viewport intersection checking methods to use standard world coordinates rather than require compatible bounds
-PPU may now be any value >0 (instead of just >1)
-UI Library is still incomplete.  TextField class is a work in progress.  Live use not recommended.

commit build id: b0000-091