Display: force a redraw after a window event.
Display: accelerate with SDL_Blit in case window size = display size.
Improve the graphical output device.

This patch migrates the Display code from SDL1 to SDL2.
Two main motivations / gained features:
- support for 1 window per Display instance
  (multiple gfx devices side by side)
- faster display rates (2D-accelerated scaled blits)

Also 4-bit indexed mode is also supported for rendered
texture.