fix: macOS stop overriding private cornerMask API to fix WindowServer GPU load spike
Electron fetched a custom `_cornerMask` for `ElectronNSWindow` to smooth
vibrancy corners. On macOS 15 (Tahoe) that private hook forces the window
shadow to be rendered from a fully transparent surface, causing the
WindowServer GPU load regression. Remove the `cornerMask` property and
the `_cornerMask` override so we stay on Apple’s default shadow path.
3a7c6dd4a5