Still quite a few things broken, especially mouse input, but video works
much better and now compiles and works on OS X too.
We now expect sdl2-config in the path and use that in the Makefile instead
of the old SDLBASE guesswork. I broke out a couple of things into
sdl_common.[ch] just to coordinate init and share the main window pointer.
Still quite some limitations in the driver, only supports RBG888 modes
right now, but adding RGB565 should be straight forward. Still quite a bit
slower than the Win/MGL driver... would like to understand why that is at
some point. But it's already much faster than the SDL1.2 version.
Signed-off-by: Kevin Shanahan <kmshanah@disenchant.net>
