[annotations] Delete fully erased annotations

[ivancea]

Problem

When erasing annotations, even after fully erasing them, they are still left as invisible selectables.

For example, given these annotations:

After erasing the 'B' letter:

Solution

When an annotation is fully erased, remove it, so it doesn't add noise when selecting.

• Render erased annotations offscreen and inspect their alpha pixels.
• Delete objects only when no meaningful pixels remain.
• Keep partially erased objects and preserve undo/redo.
• Enable per-pixel hit testing so masked areas are not selectable.
• Cover full, partial, thin-stroke, persistence, and undo/redo cases with tests.

[ivancea]

#2118 could affect this logic (Maybe?), so I'll review this after that one is merged. In case there's some weird interaction