DerelictSDL2 3.1.0-alpha.1

[Mike Parker]

SDL 2.0.6 was just released [1], so I've updated DerelictSDL2 [2] 
to support it. It's available in DerelictSDL2 3.1.0-alpha.1. I've 
tested that the loader works, but beyond that I've done nothing 
with it.

I also fixed some minor issues in the 3.0 branch, the latest 
release of which is 3.0.0-beta.7.

The 3.0 series supports SDL 2.0.0 - SDL 2.0.5, and the 3.1 series 
supports SDL 2.0.0 - 2.0.6. The latter is currently not in the 
master branch (it's in the 3.1 branch), nor is it in the 
documentation. I'll move 3.1 development to master and update the 
docs once I pull it out of alpha.

[1] https://discourse.libsdl.org/t/sdl-2-0-6-released/23109
[2] https://github.com/DerelictOrg/DerelictSDL2