Ice backdrop and screen corners draft build

Draft build of Ice with the menu bar backdrop and screen corner changes.

This build is ad-hoc signed and not notarized. macOS may require manual approval in Privacy & Security before opening it.

Known limitations:

• The bar backdrop often takes a moment to appear after switching Spaces.
• Top corner radii do not display on fullscreen apps.
• The top bar backdrop sometimes has the incorrect height on notch-less displays.