fix: clip fully occluded background dialogs

[benletchford]

Closes #608.

Summary

• invalidate front-window content clobbered while a visible window is created and then reordered behind it
• honor an empty dialog visRgn in HLE dialog redraw, text-item redraw, and retained-snapshot composition
• cover each occlusion path with focused regression tests

Validation

• cargo fmt --all -- --check
• cargo test --lib (2952 passed, 3 ignored)