initialize pixel buffer inside of makeSurface for SDL_SWSURFACE

* initialize pixel buffer inside of `SDL.makeSurface` for SDL_SWSURFACE (added test for this)
* removed https://github.com/inolen/emscripten/commit/35383300f4a420c3dbb878a83d944bb5506f7c7e#diff-52ac12f797aa53539b7d78c3dc313fadL219 . SDL_MUSTLOCK is #define'd to return 1 always
* removed some old comments describing offsets from before we had C_STRUCTS.*
* fixed potential issue in SDL_UnlockSurface where the lock count could get off if called multiple times once already unlocked

Ran browser tests.