Upgrade sdl2 to v0.9.1

[simias]

Thank you very much for that! I've been postponing the SDL2 upgrade for a while...

And I see that this would have fixed #4 on its own without any other modifications. Oh well...

[tikhomirov]

No problem! Unfortunately it won't fix #4. I'm working on OS X problems and successful build will require:

• Adding custom generator for gl crate
• Rewriting OpenGL renderer to remove all unsupported functions
• Maybe downgrading to OpenGL 3.2 if above steps will not be sufficient to fix #4

[tikhomirov]

Also you can close #3 as not needed anymore :)