Make Mac borderless windows resizable and draggable.

Despite the fact that the style mask contains `NSTitledWindowMask`, the
title doesn't show up for two reasons: (a) we draw over it; (b) we make
it invisible with a call to `-[NSWindow setTitleVisibility:]`.

Addresses servo/servo#9856 and servo/servo#9878.
Partially addresses servo/servo#9812.

src/api/cocoa/mod.rs

@@ -404,7 +404,9 @@ impl Window {
 
             let masks = if screen.is_some() || attrs.transparent {
                 // Fullscreen or transparent window
-                NSBorderlessWindowMask as NSUInteger
+                NSBorderlessWindowMask as NSUInteger |
+                NSResizableWindowMask as NSUInteger |
+                NSTitledWindowMask as NSUInteger
             } else if attrs.decorations {
                 // Classic opaque window with titlebar
                 NSClosableWindowMask as NSUInteger |