fix(wayland): map GStreamer host surface

[Lowband21]

Summary

• map the intermediary Wayland video surface with a retained transparent wl_shm buffer
• leave source cropping to GStreamer and scale only the host viewport destination
• commit synchronized viewport updates and clean up anchor resources

This keeps waylandsink’s nested subsurface tree visible on compositors that enforce recursive subsurface mapping, including KWin.

Testing

• cargo test --workspace --all-features
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo fmt --all -- --check
• Ferrex playback crate checked against the local Subwave checkout