Switch to ST_PointOnSurface

Resolves gravitystorm#1644.

This is a necessary step towards vector rendering (see gravitystorm#1644).

As this changes the label placement algorithm for polygons, this causes differences in label placement.

(SQL deliberately not indented to increase readability of the diff.)

addressing.mss

@@ -25,7 +25,6 @@
         }
       }
     }
-    text-placement: interior;
     text-face-name: @book-fonts;
     text-fill: @address-color;
     text-halo-radius: @standard-halo-radius;
@@ -56,7 +55,6 @@ no official postal addresses) */
   [zoom >= 14][way_pixels > 3000],
   [zoom >= 17] {
     text-name: "[name]";
-    text-placement: interior;
     text-face-name: @book-fonts;
     text-fill: #444;
     text-halo-radius: @standard-halo-radius;