Renamed some drawing functions, for consistency.
Refactored most of common.h into other files.
Made drawing single-threaded-only again
Some logging-related fixes.

- Fixed up log.h
- Added debug.h
- Removed error.h
Removed the need for FindDL.cmake
Minor bugfix in log.c.
Fixed a bug with vsnprintf in Windows
Font API update and added SGRect
Added sgAudioBufferCreateData and fixed a bug in sgAduioBufferSetData
Added sgSprite{Set,Get}Offset
First steps of API rework to use SGVecX.
Minor bugfix of the (temporary) compat API
Fixed sgKeyboardKey{Press,Release}.
Added .gitattributes