st: Report correct paint volumes during transitions

st: Report correct paint volumes during transitions

StWidget reports a paint volume large enough to paint the current
theme node. As CSS transitions also paint the previous theme node,
the reported paint volume may be incorrect, resulting in screen
artifacts when painting outside the reported volume.

Add st_theme_node_transition_get_paint_box() to calculate an allocation
large enough to paint both theme nodes, and use it to report the correct
paint volume during transitions.

https://bugzilla.gnome.org/show_bug.cgi?id=640085