Different handling of fullscreen option in X11 and SDL HIDD

[mattrust]

X11 HIDD has the option "--fullscreen" as command line option of AROSBootStrap SDL HIDD has "fullscreen" as icon tooltype and ReadArgs() argument.

I'm suggesting to unify that.

[Kalamatee]

IMHO, the drivers should always expose both windowed and fullscreen display modes, and use an ENV variable to disable either, or alter the behaviour (chose FULLSCREEN over Windowed for instance)