[cmdocmd]

Description

Fixes jitter in stationary filtered content when an animated mask changes bounds.

Mask children are excluded from render bounds so their animation does not shift the bitmap cache origin. Script-visible bounds remain unchanged.

Testing

Adds visual/cache_as_bitmap/animated_mask_filter_bounds, using an existing Ruffle test image and references from Adobe Flash Player 32.

The test covers:

• Stationary content under oversized animated masks, with ColorMatrix, Hard Light, both effects, or neither.
• An intentionally moving clipping edge.
• An intentionally moving bitmap.

It compares traces of bitmap, mask, and container bounds against Flash. Image comparisons at three animation checkpoints and the final state verify that stationary content stays still while clipping and content movement update correctly.

The capture checkpoints have identical visible control positions but different oversized mask bounds, allowing the same Adobe stage reference to be reused.

The test passes with both changes and fails when either change is removed.

Checklist

• I, a human, have self-reviewed this PR and fully understand the changes within.
• I have made or updated tests where possible.
• All of my commits are properly scoped, compile successfully, and pass all tests.
• This PR does not make sense to split up into smaller PRs.
• An LLM was involved in the authoring of this code.