[Tilecache] Quick panning with overlays

[Yves Moisan]

> maybe the right question would be: And how many overlay layers do you
> have Yves?
> 
> I remember one OL user was complaining about speed, and he had like 80
> layers :D Keeping it under 3 layers at a time should do the trick i
> think. 

Well indeed that's an issue.  I have 20 some overlays.  I know it's too
much and a lot of those layers could be turned into composite layers
e.g. all line feeatures could be grouped in one layer with symbology,
but that means using a custom LayerSwitcher.

> People should take more care with number of layers. Instead of
> piling up the layers, one should group layers in wms request: you
> loose some speed on rendering it, but if you use caching its well
> worth it ;)

The overlays are not cached.  I'm supposed to have a script that bundles
calls to MapServer somewhere.  If you have pointers, I'll take them.

Thanx,

Yves