Basic test implementation of an ice flag. Allows for slick parts of terrain. Intended for ice gun, or "ice" mask on portions of land objects.
In this test variant it is triggered on girders/objects/bridges of the snow/christmas theme, or on a map that uses blue as a mask colour. Probably needs sheepluva's slope detection to make slopes more slippery to climb.
306cedbeb213
fixed several 'retina display' images and added a bunch more (though just from upscaling the normal ones; might be nice to have 'em redrawn at a real higher resolution)
