Platform: fixed Sdl2Application::tryCreateContext().
Platform: fixed Sdl2Application::tryCreateContext().

It seems that the context is negotiated when creating the window, thus
it fails there and not later when actually creating the context.

DerThorsten/magnum,MiUishadow/magnum,MiUishadow/magnum,DerThorsten/magnum,ashimidashajia/magnum,ashimidashajia/magnum,DerThorsten/magnum,DerThorsten/magnum,ashimidashajia/magnum,DerThorsten/magnum,MiUishadow/magnum,ashimidashajia/magnum,ashimidashajia/magnum,MiUishadow/magnum,MiUishadow/magnum,MiUishadow/magnum