libyaml-cpp: add versions 0.5.1 and 0.3.0

The two versions are actively maintained, and the API changed between them.
SDL2: add release candidate

The tarball is not marked as such, but current 2.0.0 version is still a release
candidate for SDL2.
Add SDL2_image and SDL2_mixer, release candidate.
Add SDL2_gfx (svn version)

There is no package yet, adding latest revision.
SDL2_gfx: add simlinks for headers with SDL_*

All the SDL2 libraries distribute headers with SDL_* names, under SDL2. Do the
same for the gfx one. I told upstream, but no changes so far.
SDL2: update hashes for new stable release tarballs.

I also had to update the url to the main SDL2 lib.
Merge branch 'sdl2' of https://github.com/cpages/nixpkgs