Add st_theme_node_paint_equal() and use that to do two things:
1) Avoid animating transitions where nothing changes.
2) Copy cached painting state from the old theme node to the new
theme node.
https://bugzilla.gnome.org/show_bug.cgi?id=627083
The current CSS3 border-image is close to a superset of what we were
doing for -hippo-background-image. Woot! rename StThemeImage to
StBorderImage and change parsing to look for:
border-image: <url> <number>...
Rather than
-st-background-image: <url> <length>...
percentanges for the border sizes are not currently supported, neither
are the keywords for handling of the middle part. We always do 'stretch'
for now.
https://bugzilla.gnome.org/show_bug.cgi?id=595990
