app: update drawable bounding box when floating-selection bounding box changes

When the bounding box of a floating selection changes, update the
bounding box of the associated drawable, since floating selections
are no longer clipped to the drawable's bounds in general.
5 jobs for master in 24 minutes and 42 seconds
Status Job ID Name Coverage
  Babl
passed #453215
babl-git

00:06:10

 
  Gegl
passed #453216
gegl-git

00:02:51

 
  Gimp
passed #453217
build-git-autotools

00:12:36

passed #453218
build-git-meson

00:04:42

passed #453219
build-git-x86_64-w64-mingw32-meson

00:15:38