Make it possible to draw the titlebar and clip the window in HTML

[paulrouget]

For browser.html, we want to draw the titlebar ourselves.

On osx, we need to be able to draw a window with rounded corners. the browser.html HTML page would use html {border-radius:6px}. To support that, we need to:

• add an option to start servo with no titlebar
• create the glutin window with with_decorations(false).with_transparency(true);
• in rust-layers, render_scene should use a transparent clear color;
• glutin needs to expose a invalidate_window_shape method, or something like that, that would call the cocoa method window.invalidateShadow() to invalidate the shadow once the border radius has been painted. Edit: see Make it possible to draw the titlebar and clip the window in HTML #7659 (comment)

With all of this, it appears to work:

There are some more issues (the border radius doesn't clip its content, see #7575, and for some reasons vertical coordinates are messed up), but these can be fixed later.

Can someone tell me if this approach makes sense?

Letting HTML define the shape of the window is nice, but it's not a hard requirement. Maybe instead of using border-radius, we can clip the window in the compositor directly, or expose a function in Glutin to clip the window, or maybe always add rounded corner to decoration-less windows.

PS: we will need more things to make the titlebar fully functional:

• setting the dragging region with CSS: Support setting the window dragging region with CSS #7408
• support closing, maximizing and minimizing the window from JS (window controls will be built in HTML/CSS). This will be possible with Implement a Servo-based runtime #7379

[paulrouget]

Started working on this.

Found a better way to properly draw decoration-less cocoa windows: rust-windowing/glutin#606

[paulrouget]

Fixed by #7778 and rust-windowing/glutin#606.