SDL: Implement stretch mode API

Four modes are supported:
 - Use original size with no scaling
 - Scale by an integral amount as much as possible but not bigger
   than the window.
 - Scale to fit the window while respecting the aspect ratio. There
   may be black bars on the left and right, or on the top and bottom,
   but not both. This is the default, and the old behaviour.
 - Scale and stretch to fit the window. In this mode the aspecy ratio
   is not respected and there is no black bars.
The mode is controled by the "scaling_mode" value (between 0 and 3) in
the config file.

Also add Crtl-Alt-s hotkey to cycle through scaling modes

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