New Features:
- FNAPlatform: Add GetBaseDirectory function for upcoming platform support
- SpriteBatch: Reorganize variables to make SpriteBatch.cs easier to re-use outside of FNA
Fixes:
- Effect: Allocate state changes struct as AllocHGlobal memory, fixes crash on Apply
- Video: Add width/height information to XNB mismatch error message
- SoundEffect: Fix FromStream loading for MSADPCM wav data
- SoundEffectInstance: Allow using Apply3D(AudioListener[]) with one listener
- SDL2_FNAPlatform: Fix event filter for Windows SDL_WINDOWEVENT_EXPOSED events
- FNA.Core: Fix building in the same directory as FNA