snow app: calculate defaults for screen size

[rinusser]

Currently the "snow.efi" application works and looks as intended in the default text mode. However with increasingly larger screen sizes (in terms of rows/columns) the default values result in too few, too slow moving flakes with a clear boundary between the initial set and the first generation of new flakes. Additionally, wind works, but is too slow for those larger sizes.

Change the defaults so that they depend on the screen size. Larger screen sizes should default to more, faster moving flakes.