Add map->setSize() to PHP MapScript

[dmorissette]

In bug 836, issues were reported with mapscript code setting map->width and
map->height directly without calling msMapComputeGeoTransform(). A new
mapObj.setSize() method was added to address that in SWIG MapScript and we
should add the same to PHP MapScript to bring them in sync.

However I'm not sure this method was really needed since AFAIK all other calls
in MapScript that use the width/height end up indirectly calling
msMapComputeGeoTransform().

Let's wait to hear feedback on bug 836 comment 8 before adding this to PHP
MapScript.

[dmorissette]

Setting FUTURE target milestone.

[dmorissette]

Frank and Sean have convinced me that it's a good thing to switch to using a
setSize() method instead of setting width/height directly.

Setting width/height directly will continue to work, but using setSize() instead
will be recommended in the docs.

[dmorissette]

Done. map->setSize() added to PHP MapScript.