st-theme-node-drawing: Move most of the cached paint state to another struct

st-theme-node-drawing: Move most of the cached paint state to another struct

Since we now share theme nodes between, we shouldn't cache the paint state
across all nodes. As a first step towards putting this in the actor, split
out the state into another structure. Keep it in the theme node for now
so that we don't make too many changes in one commit.

It's possible that some of these pieces of drawing state could be shared
between theme nodes. For the sake of simplicity, assume that none of them
are shared or should be shared. A future commit could identify those that
could be shared and move them back into the theme node.

https://bugzilla.gnome.org/show_bug.cgi?id=697274