- Fixes bug 1712 by not overwriting SDL_SubsystemRefCount in SDL_Init.
- Removes the SDL_initialized variable, and makes SDL_SubsystemRefCount
the canonical source of truth for whether or not a subsystem has been
initialized.
- Refactors SDL_InitSubSystem and SDL_QuitSubSystem to use helper
functions to manage refcount.
- Adds automated tests for SDL_Init/Quit*.
- Adds SDL_bits.h which contains SDL_MostSignificantBitIndex.
