stylecontext: Change fallback behavior on state mismatch

stylecontext: Change fallback behavior on state mismatch

For functions that take state flags as an argument we need to special
case the situation where the passed in flags don't match the current
state.

Previously we would create a copy of the style info, change its state
and do the lookup from there.

Now that GtkCssNode has replaced style infos, this doesn't work as well
anymore as copying a GtkCssNode is not possible.
However, unike style infos, GtkCssNodes are instant-apply, so we don't
need to copy anymore, we can just change the state of the node.

This causes some invalidations to be queued, but we can take that
performance hit as this is fallback code.

https://bugzilla.redhat.com/show_bug.cgi?id=1228852