Debug build should start windowed by default

[Forestidia86]

A debug build is often used for debugging purposes but these can be impeded by the default software fullscreen, e.g. since you can't access crash messages or don't get out of the program window easily.
So at least the debug build should default to windowed mode. 

[Simon]

Very sensible, I made issue #369.

If you already have a user file, its options will continue to override the default.

-- Simon