window/wayland: Calculate bottom and right frame extents
The bottom and right frame extents were never calculated and thus always remained 0. This did not lead to any obvious problems until 6cbc5180 which started relying on those to calculate the buffer rect. This resulted for example in window screenshots being cut off at the bottom right corner of the window rather than the buffer.
Fixes: 6cbc5180 Closes: gnome-shell#6050 (closed)