Adding run-time constants to common expression handling.

[sdlime]

Basically it would be nice to expose a few run-time constants to MapServer logical expressions. Things like computed extent, scale, image size, and cellsize. Syntax would be something like !name! following after the !bbox! hack in PostGIS. While I’m only interested in MapServer logical expressions it’s reasonable that certain drivers could support them as well within their DATA/FILTER statements.

I’m proposing !bbox! (as define in PostGIS code), !minx! (etc…), !scale!, !width!, !height!, and !cellsize!. One use for this would be triggering scaling on parameters passed to style GEOMTRANSFORMs (e.g a buffer).

Steve