Add SDL hardware renderer

This renderer uses the GPU-based SDL_Texture the is part of SDL 2. To
use the renderer, launch Flare with:

    flare --renderer=sdl_hardware

Known issues:
- Text looks "narrower" than with the SDL software renderer. This is
because the software renderer uses `SDL_gfxBlitRGBA()`, which uses
a non-linear alpha table to make things appear brighter.