Always use getAABB for fence bounds

[adroitwhiz]

Resolves

Resolves #477 and resolves #446

Proposed Changes

This PR changes the getFenceBounds function to use getAABB for all types of skins, as opposed to just bitmap skins.

Reason for Changes

getFastBounds does different things depending on external state, leading to inconsistent behavior.
It is also incompatible with Scratch 2.0, which fences all sprites, including vector ones, based on their AABB.

Test Coverage

I've added a fencing test which ensures fencing is the same before and after "if on edge, bounce" (which calls getBounds internally) is called.

[thisandagain]

/cc @cwillisf @kchadha @BryceLTaylor

[cshaa]

Ping! What is the current status of this PR?

[fsih]

Testing:

• Make sure to test games where you switch levels/die by walking off the screen
• Clicking on a sprite in a sprite pane, using the if on edge bounce block, used to cause the bounding box of the sprite to shrink by ~half a pixel, but now it should NOT change.

[fsih]

LG! Thanks!