Draw black corners behind Maximized or Tiled windows

[cassidyjames]

Problem

Rounded LibHandy windows look good now when tiled because we're drawing black corners in the Stylesheet, but when going into the overview or any sort of window animation, you end up seeing these corners.

Tiled	Overview

Proposal

We should draw these black corners in Gala, not the Stylesheet, so we can only show them in the normal state and not in Multitasking View or any window animations.

[danirabbit]

This is case for all windows that are maximized or tiled

[cassidyjames]

Ah yeah, I guess we square off "inside" corners of regular windows when tiled, but the outside corners are still rounded and should get the black masking when not in the overview/animations.