gtk_widget_translate_coordinates() doesn't have to always set the
dest_x argument. Let's initialize the argument before use to prevent
usage of uninitialized value, which also prevents "Conditional jump
or move depends on uninitialised value(s)" messages from valgrind.
When animations are disabled, the @keyframes CSS rule doesn’t work, and
neither does animation-name — so we need to set the margin-left to the
arrow’s final location directly.
Signed-off-by: Philip Withnall <withnall@endlessm.com>
