Various SDL fixes:

Pass window size in SDL_USEREVENT_SETVIDMODE
- Fixes race condition where SDL_WINDOWEVENT_RESIZED or
  SDL_WINDOWEVENT_SIZE_CHANGED arraves while
  SDL_USEREVENT_SETVIDMODE is in the queue

Set new scaling based on old scaling multiplier
- Will resize window on mode changes, but tries to keep integer
  multiplier similar.  May break fullscreen modes, dunno.

Have aspect_fix() return higher rather than lower
- This may make it the same as aspect_correct(), but I'm too lazy
  to figure that out.