[mapserver-dev] Implement msPostgisLayerGetExtent

[Even Rouault]

Just an idea :

Similarly to ( http://mapserver.org/mapfile/projection.html#projection-auto )
PROJECTION
	AUTO
END

why not having ?
EXTENT AUTO

When this is defined, it would go to the possibly expensive computation.

So we would have :
- no EXTENT specified at layer level
- EXTENT xmin ymin xmax ymax
- EXTENT AUTO

The question is when no EXTENT is specified, should that be equivalent to 
"EXTENT AUTO" for layers where it is cheap such as shapefiles ? We would 
probably a virtual method HasFastExtentComputation to do that

Even

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