Previous workaround wasn't sufficiently stable. Infinite looping could be
triggered if the user set:

panel size = 16
panel rows = 1
workspace rows = 1
no of workspaces = 100

and then started increasing/decreasing panel size by 1.

This fix rearranges the code (storing last pager height instead
of its aspect ratio) and introduces a minimum WNCK panel height of 10px.
Tbis makes it stable in the above scenario.

Still, we should push for a real fix in the WNCK pager:
https://bugzilla.gnome.org/show_bug.cgi?id=664779