macos: add more aggressive clip to transparent subview

macos: add more aggressive clip to transparent subview

The Cairo implementation for the Macos backend uses a toplevel
window with full transparency and a series of NSView to create opaque
regions. This improves compositor performance because it allows the
display server to avoid costly blends.

However, we want to ensure we clip better when exposing the
transparent region so that we only expose the shadows/corners as
necessary.