pixelcache: reuse existing timeout source when possible

pixelcache: reuse existing timeout source when possible

This avoids the g_source_remove(), g_source_destroy(),
g_timer_source_new(), and g_source_set_name_by_id() in the common case.

Instead, we reuse our previous source and update the ready time to our
new deadline. We lose the coalescing with g_timeout_add_seconds(), but
that is not going to help in the common case anyway (unless you have
three hands and can scroll multiple pixelcached backed widgets at once).

https://bugzilla.gnome.org/show_bug.cgi?id=765640