macOS: fix opacity handling

Not using `NSColor::clearColor()` results in Quartz thinking that the
window is not transparent at all, which results in artifacts.

However, not setting the `windowBackgroundColor` in
`Window::set_transparent` results in border not properly rendered.

Fixes: 94664ff687 (Don't set the background color)

Cargo.toml

@@ -142,6 +142,7 @@ features = [
     "NSApplication",
     "NSBitmapImageRep",
     "NSButton",
+    "NSColor",
     "NSControl",
     "NSCursor",
     "NSDragging",