Added PixelRatioForZoom function to replace

We had a badly named function called PixelsToNative
that did not do a conversion but returned a ratio.

Renamed and expanded to all any tile dimension.

slippy/tile.go

@@ -143,3 +143,28 @@ func FromBounds(g TileGridder, bounds geom.PtMinMaxer, z Zoom) ([]Tile, error) {
 
 	return ret, nil
 }
+
+// MvtTileDim is the number of pixels in a tile
+const MvtTileDim = 4096.0
+
+// PixelRatioForZoom returns the ratio of pixels to projected units at the given zoom. Multiply this value by
+// the pixel count in tile.buffer to get the expected conversion.
+//
+// if zoom is larger the MaxZoom, it will be set to MaxZoom
+// if tileDim is 0, it will be set to MvtTileDim
+func PixelRatioForZoom(g TileGridder, zoom Zoom, tileDim uint64) float64 {
+	if zoom > MaxZoom {
+		zoom = MaxZoom
+	}
+	if tileDim == 0 {
+		tileDim = MvtTileDim
+	}
+	ext, _ := Extent(g, Tile{Z: zoom})
+	return ext.XSpan() / float64(tileDim)
+}
+
+// MvtPixelRationForZoom returns the ratio of pixels to projected units at the given zoom.
+// This assumes an MVT tile is being used.
+func MvtPixelRationForZoom(g TileGridder, zoom Zoom) float64 {
+	return PixelRatioForZoom(g, zoom, MvtTileDim)
+}