Wayland: Fix erratic fallback decoration behavior

The handler for xdg_toplevel::configure treated the provided size as the
content area size when instead it is the size of the bounding rectangle
of the wl_surface and all its subsurfaces.

This caused the fallback decorations to try positioning themselves
outside themselves, causing feedback loops during interactive resizing.

Fixes #1991
Fixes #2115
Closes #2127
Related to #1914

CONTRIBUTORS.md

@@ -24,6 +24,7 @@ video tutorials.
  - Waris Boonyasiriwat
  - Kyle Brenneman
  - Rok Breulj
+ - TheBrokenRail
  - Kai Burjack
  - Martin Capitanio
  - Nicolas Caramelli
@@ -145,6 +146,7 @@ video tutorials.
  - Pierre Moulon
  - Martins Mozeiko
  - Pascal Muetschard
+ - James Murphy
  - Julian Møller
  - ndogxj
  - n3rdopolis