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

(cherry picked from commit 0f5b095042)

CONTRIBUTORS.md

@@ -23,6 +23,7 @@ video tutorials.
  - Waris Boonyasiriwat
  - Kyle Brenneman
  - Rok Breulj
+ - TheBrokenRail
  - Kai Burjack
  - Martin Capitanio
  - Nicolas Caramelli
@@ -143,6 +144,7 @@ video tutorials.
  - Jon Morton
  - Pierre Moulon
  - Martins Mozeiko
+ - James Murphy
  - Julian Møller
  - ndogxj
  - Kristian Nielsen