GameCanvas supports efficient colour-coded collision masks

GameCanvas should support an efficient, colour-coded collision mask for platformer-style terrain. A game should be able to load a mask once and sample it at world coordinates from per-frame foot or body probes. Each colour can identify a terrain type—such as solid ground, slopes, ladders, water, hazards, or one-way platforms—so games can implement smooth slopes and surface-specific behaviour without querying Picture pixels every frame or treating an entire tile as solid.