stylecontext: Make font hack not crash

stylecontext: Make font hack not crash

It's a lot uglier now, but it shouldn't crash anymore.

We must update the font description for animations, but we can't free it
on query, because some paths call gtk_style_context_get_font() twice in
a row without stopping the use of the first call. So us just creating a
new font description all the time and unreffing the old one is not a
good idea. So we just mere the new one into the old one.

https://bugzilla.gnome.org/show_bug.cgi?id=691186