Update .gitignore
Add SDL2 runtime binaries
Initial migration to SDL2

Leaving i_video.c to last, since that's where the main code changes are.
Make SDL2 support a separate solution config

So I can easily switch between the two. I likely won't get SDL2 out
before the next release.
Further changes to migrate to SDL2

Extensive changes to i_video.c. It compiles now, and runs, but no
pillarboxes, and it crashes when going widescreen and with some
keypresses. There's a lot more work to do.
More work on i_video.c to get SDL2 working
Merge pull request #42 from bradharding/SDL2

Merge SDL2 branch