Surface-based destructible terrain

For an hypothetic nonexistent reader who can't figure the twenty lines of code out, this simply blits a circle of X color onto a surface whose colorkey was previously set to X.

Coming up: destructible terrain based on rectangle subdivision. It's better than the surface approach if you actually want this terrain to be useful for stuff like checking whether a platformer character is on it.

The spiffy terrain images are GIMP-powered. Clouds are brushes on a gradient, terrain is brushes, smudges, and unsharp masking.