Part 7 - collision detection against platforms, snowball AI using them

The screen is divided into 16px wide columns (0..15, 16..31, ...), each column has list of platforms (their Y coordinates) and there's new routine GetPlatformPosUnder which will return the platform under the current sprite.

And the snowball AI was modified to use the new function and make the snowballs roll down through the level until they get out of screen completely.