GtkWindow: Fix the shadow width logic

GtkWindow: Fix the shadow width logic

Previous commit 305b34a "GtkWindow: fix move/get position with CSD"
introduced a regression because some windows presumably use shadows but
actually don't, resulting in a negative offset being wrongly applied.

Problem is that get_shadow_width() would return non-zero shadows even
for windows that have no shadow, thus causing the negative offset.

Fix the logic in get_shadow_width() and gtk_window_should_use_csd() so
that get_shadow_width() returns accurate values.

Bugzilla: https://bugzilla.gnome.org/show_bug.cgi?id=756618