csd: Avoid jumping windows

csd: Avoid jumping windows

The size of the shadow and invisible borders can (and usually
will) change between backdrop and focused windows, while the
overall window size remains unchanged. This causes the visible
window to visually 'jump'. We can avoid this by always reserving
the maximum of the focused and unfocused border sizes. The code
for positioning the input-only windows making up the invisible
border is adjusted to deal with this. We now always place the
invisible border right outside the visible content, even if the
shadow extends out much farther.

https://bugzilla.gnome.org/show_bug.cgi?id=707524