Make sure we don't unnecessarily update the group layer's drawable
while flusing the group's projection during resizing, since we want
to either update the entire drawable, or avoid any updates, when
replacing the drawable's buffer.  Note that explicitly supressing
updates in this case should theoretically not be necessary, but the
fact that the call to gimp_projectable_bounds_changed() can result
in reconstructing the projection (see the FIXME comment in that
function) makes it necessary in some cases nonetheless.

(cherry picked from commit bd726c96)