ADPCM Support, minor graphic fixes

The primary purpose of the request is to add ADPCM support for WaveBanks. Basically just implemented it with my public domain decoder... with some fixes that I need to merge into the library, heh.

This also fixes a few crashes, namely the OpenTKGameWindow crashing on a null context (though it sounds like the issue is deeper than a null reference?) as well as a bizarre operation in SpriteBatch, where we were doing needless math to the passed `origin` parameter.

Other than that, just a couple stubbed methods that are needed for XNA4 compliance.