SDL: Only recreate SDL2 window when necessary

Destroying and recreating the SDL window whenever the video mode
changes in SDL2 is not necessary and causes several problems:

1. In windowed mode, the game window shifts position;
2. In fullscreen mode in macOS, every time the window is
   recreated, it causes the OS to play its switch-to-fullscreen
   animation again and emit system alert noises;
3. The window content flickers; and
4. The engine loses events from the old destroyed window.

This patch changes the SDL backend code to avoid destroying and
recreating the SDL window when using SDL2, except when switching
OpenGL modes, since there is no way to change the OpenGL feature
of a window.

There are still some outstanding issues with OpenGL where window
size ends up getting reset even though the user has resized it;
this will probably need to be addressed at some point in another
patch.

Thanks to @bgK and @criezy for their feedback which made this
patch much better.

Co-Authored-By: Bastien Bouclet <bastien.bouclet@gmail.com>

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