allow -geometry to specify fullscreen or windowed modes, like PrBoom

This allows strings like "640x480w" to cause the engine to run in a
window, while strings like "1280x800f" cause the engine to run in
fullscreen mode.  This is inspired by the behavior of the PrBoom
-geometry parameter.
i_video: fix an if condition in my previous commit

`else if (f == 'w')` doesn't really secure itself against
uninitialized memory, it still needs to test that `s == 3`.
Merge pull request #447 from chungy/master

allow -geometry to specify fullscreen or windowed modes, like PrBoom