SDL: do something with SDL_GL_GetDrawableSize

[Frenzie]

For Wayland type stuff where they seem to have the odd concept that a window will think it's 800x600 even though it's displayed at 1600x1200 (or whatever). Unless I'm overlooking something it's not just as simple as declaring that you'll resize nicely on any size (which I thought we already did with the allow hidpi flag).
https://wiki.libsdl.org/SDL_GL_GetDrawableSize