Add pixel_scale (#230)

openmaptiles.yaml

@@ -23,6 +23,7 @@ tileset:
   center: [-12.2168, 28.6135, 4]
   maxzoom: 14
   minzoom: 0
+  pixel_scale: 256
   defaults:
     srs: +proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over
     datasource: