Switch to RGB565 pixel format.

With RGB555, there were green artifact pixels on slopes adjacent to water.
Use a separate Makefile stanza for sdl and sdl2, since they are in fact different enough.
Misc cleanups:

- Improve some casts
- Remove some dead code
- Remove the width alignment code unless it demonstrates itself to be necessary.
Add sdl2 backend to config.template
- Use SDL_EventState to filter out events (thanks TurfIt)
- Remove SDL_INIT_NOPARACHUTE unless it demonstrates itself to be necessary.
Fix sound stuckness in SDL2.