Platform: try to create core context in Sdl2Application on all platforms.

Originally enabled just on OSX, apparently something similar is needed
also in Mesa. If the version is not user-specified, core GL 3.2 is
created on OSX, core 3.0 elsewhere. If that fails, the application falls
back to creating compatibility 2.1 context.

Hopefully I didn't break anything. The only difference on OSX is that
the application doesn't fall back anymore if the version is
user-specified.