Add new map provider OpenSeaMap & overlay example (#114)

* Add new map provider OpenSeaMap & overlay example

* better comment

---------

Co-authored-by: Florian Pigorsch <fpigorsch@gmail.com>

examples/overlay/main.go

@@ -0,0 +1,31 @@
+// This is an example on how to use a map overlay.
+
+package main
+
+import (
+	sm "github.com/flopp/go-staticmaps"
+	"github.com/fogleman/gg"
+	"github.com/golang/geo/s2"
+)
+
+func main() {
+	ctx := sm.NewContext()
+	ctx.SetSize(1600, 1200)
+
+	ctx.SetCenter(s2.LatLngFromDegrees(48.78110, -3.59638))
+	ctx.SetZoom(15)
+
+	// base map
+	ctx.SetTileProvider(sm.NewTileProviderOpenStreetMaps())
+	// OpenSeaMap as a overlay to the base map
+	ctx.AddOverlay(sm.NewTileProviderOpenSeaMap())
+
+	img, err := ctx.Render()
+	if err != nil {
+		panic(err)
+	}
+
+	if err := gg.SavePNG("overlay.png", img); err != nil {
+		panic(err)
+	}
+}

tile_provider.go

@@ -122,6 +122,17 @@ func NewTileProviderOpenCycleMap() *TileProvider {
 	return t
 }
 
+// NewTileProviderOpenSeaMap creates a TileProvider struct for OpenSeaMap's tile service
+func NewTileProviderOpenSeaMap() *TileProvider {
+	t := new(TileProvider)
+	t.Name = "sea"
+	t.Attribution = "Maps and Data (c) openstreetmaps.org and contributors, ODbL"
+	t.TileSize = 256
+	t.URLPattern = "http://t1.openseamap.org/seamark/%[2]d/%[3]d/%[4]d.png"
+	t.Shards = []string{}
+	return t
+}
+
 // NewTileProviderCarto creates a TileProvider struct for Carto's tile service
 // See https://github.com/CartoDB/basemap-styles?tab=readme-ov-file#1-web-raster-basemaps for available names
 func NewTileProviderCarto(name string) *TileProvider {
@@ -179,6 +190,7 @@ func GetTileProviders(thunderforestApiKey string) map[string]*TileProvider {
 		NewTileProviderOpenTopoMap(),
 		NewTileProviderOpenStreetMaps(),
 		NewTileProviderOpenCycleMap(),
+		NewTileProviderOpenSeaMap(),
 		NewTileProviderCarto("rastertiles/voyager"),
 		NewTileProviderCartoLight(),
 		NewTileProviderCartoDark(),