Convex hulls should be calculated in texture space, not "native size" space

[adroitwhiz]

Expected Behavior

To ensure maximum accuracy, skins' convex hulls should be calculated in texture-space by going over each texel. This will also ensure fencing compatibility with Scratch 2.0.

Actual Behavior

Currently, convex hulls are generated at the "native size" of the skin. For bitmap skins, this is half their resolution, and that leads to certain inaccuracies when generating convex hulls for bitmap costumes.

Steps to Reproduce

Currently, there are enough existing issues with the way "silhouettes" work (convex hulls are generated from "silhouettes") that it's probably impossible to trace any specific issue back to this bug. Once #479 goes in, I can provide some concrete examples.

Operating System and Browser

All