Non-clipped polygon annotation

[project10]

Hi devs,

I've implemented support for an item that was discussed on IRC last week 
- annotating polygons based on their pre-clip shape. This gives fixed 
label positioning for polygon layers without the need to precompute an 
annotation point based on the polygon centroid.

Frank suggested that I commit my ideas to a proper RFC format, but I 
wanted to first get some feedback from you guys.

The patch adds support for a NOCLIPANNO processing directive at the 
layer level. If the processing option is enabled, it will copy the 
polygon shape pre-clip and use that to determine annotation points. If 
not given, the old behavior is used.

My initial implementation is at http://rafb.net/p/4wGaHV10.html

Some renders showing the behavior change:
http://project10.net/ms/normal.png - current behavior

http://project10.net/ms/normal-myissue.png - Why I didn't like the 
current behavior. My labels get 'dragged around'.

http://project10.net/ms/annoclip-inview.png - Labeling of a polygon with 
my change

http://project10.net/ms/annoclip-demo.png - If the map is panned, the 
annotation point stays fixed, even though part of the shape it annotates 
is now clipped.

http://project10.net/ms/annoclip-partials.png - It also plays nice with 
partials.