Let the window be resizable
Created by: vlag
This issue concern a suggestion to make the window manually resizable.
Now that we set an explicit render context size and window size, any mode can have any size without deformation (we can add the SDL_WINDOW_RESIZABLE flag to the window right now), so maybe we can envisage to remove the WINDOWED_STRETCHED mode, and use an unique default_size of 640x480 (or quest_size value) for all modes, instead of the mode_sizes[] array.