Use pixel centers in WebMercatorGridPointSet

All other uses of pixel edge functions now seem to be only for finding
bounds of pixels, the entire grid, or extents. Point positions should
now be uniformly in the center of Mercator pixels.

src/main/java/com/conveyal/r5/analyst/WebMercatorGridPointSet.java

@@ -12,6 +12,8 @@
 
 import static com.conveyal.r5.analyst.Grid.latToPixel;
 import static com.conveyal.r5.analyst.Grid.lonToPixel;
+import static com.conveyal.r5.analyst.Grid.pixelToCenterLat;
+import static com.conveyal.r5.analyst.Grid.pixelToCenterLon;
 import static com.conveyal.r5.analyst.Grid.pixelToLat;
 import static com.conveyal.r5.analyst.Grid.pixelToLon;
 import static com.conveyal.r5.common.GeometryUtils.checkWgsEnvelopeSize;
@@ -115,14 +117,14 @@ public double sumTotalOpportunities () {
 
     @Override
     public double getLat(int i) {
-        long y = i / this.width + this.north;
-        return pixelToLat(y, zoom);
+        final int y = i / this.width + this.north;
+        return pixelToCenterLat(y, zoom);
     }
 
     @Override
     public double getLon(int i) {
-        long x = i % this.width + this.west;
-        return pixelToLon(x, zoom);
+        final int x = i % this.width + this.west;
+        return pixelToCenterLon(x, zoom);
     }
 
     @Override