SDL 2.0 support

As discussed on IRC, this adds configurable libsdl2 support to MegaZeux. It should improve portability and keep MegaZeux on life-support for a while longer. SDL 2.0 went RC in May, and the final release is due shortly. Debian and other distributions are already packaging SDL 2.0. You build with --enable-libsdl2 to get the support. It defaults off.

All of the renderers except overlay1 and overlay2 are working. The overlay renderers may never work with SDL 2.0, because it does not support YUV overlays natively any longer. I might try to add primitive blit support to them to keep them working, but that can happen later.

Input should be fully working, however SDL has historically had a lot of bugs in its input support, and they changed quite a lot of things in SDL 2.0. There is probably more work to do here, but I was able to use the robot editor, so I think it's good enough for now.

I have also updated the mingw32.zip dependency package under scripts/deps for win32 only.