postgis memory allocation patches

[tbonfort]

attached is a small patch on mappostgis.c that cuts down the number of memory allocations that occur in msPostgisNextShape(), wkbConvPolygonToShape(), wkbConvLineStringToShape(), wkbConvPointToShape() and wkbReadLine()

In msPostgisNextShape() we use a static string for *wkb, and only resort to allocating *wkb if the returned line is larger than the static string.

in the wkbXXX() functions, we use a static lineObj instead of allocating it at each usage.

On my testcase where a postgis query is returning a high number of features, the processing time in in msPostgisLayerNextShape passed from 712ms to 518ms. The total allocated memory went from 205MB to 91MB

[pramsey]

Look fine to me, feel free to apply.