fix(wayland): implement content fit geometry

[Lowband21]

Summary

• map Iced Contain, Cover, Fill, None, and ScaleDown modes to GStreamer's Wayland render rectangle
• update video geometry when the fit mode changes without shrinking the background or subtitle canvas
• keep PGS composition aligned with the fitted video rectangle, including cover clipping
• preserve zero-copy rendering and keep the transparent mapping anchor unscaled and untagged

Validation

• cargo fmt --all -- --check
• cargo test --workspace --all-targets --locked
• cargo clippy --workspace --all-targets --locked -- -D warnings
• Ferrex --features mpv integration check in its Nix development shell using the local Subwave patch