Fix onStyleImageMissing not fired in Snapshotter (#317)

* Modify example to trigger OnStyleMissing callback

* Fix onStyleImageMissing callback not works in snapshotter.

* Update gl-native

* Update doc

* Fix android check

MapboxGLAndroidSDK/src/main/java/com/mapbox/mapboxsdk/snapshotter/MapSnapshotter.java

@@ -108,9 +108,18 @@ public class MapSnapshotter {
   public interface Observer {
 
     /**
-     * Called when snapshotter finishes loading it's style.
+     * Called when snapshotter finishes loading its style.
      */
     void onDidFinishLoadingStyle();
+
+    /**
+     * Called when the map is missing an icon.The icon should be added synchronously with
+     * {@link MapSnapshotter#addImage(String, Bitmap, boolean)} to be rendered on the current zoom level.
+     * When loading icons asynchronously, you can load a placeholder image and replace it when you icon has loaded.
+     *
+     * @param imageName the id of the icon that is missing
+     */
+    void onStyleImageMissing(String imageName);
   }
 
   /**
@@ -513,7 +522,7 @@ public class MapSnapshotter {
    * @param bitmap the pre-multiplied Bitmap
    * @param sdf    the flag indicating image is an SDF or template image
    */
-  private void addImage(@NonNull final String name, @NonNull Bitmap bitmap, boolean sdf) {
+  public void addImage(@NonNull final String name, @NonNull Bitmap bitmap, boolean sdf) {
     nativeAddImages(new Image[] {toImage(new Style.Builder.ImageWrapper(name, bitmap, sdf))});
   }
 
@@ -797,7 +806,9 @@ public class MapSnapshotter {
    */
   @Keep
   protected void onStyleImageMissing(String imageName) {
-    onSnapshotFailed("style image is missing: " + imageName);
+    if (observer != null) {
+      observer.onStyleImageMissing(imageName);
+    }
   }
 
   private void checkThread() {

MapboxGLAndroidSDKTestApp/src/main/java/com/mapbox/mapboxsdk/testapp/activity/snapshot/MapSnapshotterActivity.java

@@ -138,7 +138,8 @@ public class MapSnapshotterActivity extends AppCompatActivity {
 
       // marker source
       FeatureCollection markerCollection = FeatureCollection.fromFeatures(new Feature[] {
-        Feature.fromGeometry(Point.fromLngLat(4.91638, 52.34673), featureProperties("2", "Car"))
+        Feature.fromGeometry(Point.fromLngLat(4.91638, 52.35673), featureProperties("1", "Android")),
+        Feature.fromGeometry(Point.fromLngLat(4.91638, 12.34673), featureProperties("2", "Car"))
       });
       Source markerSource = new GeoJsonSource(MARKER_SOURCE, markerCollection);
 
@@ -156,17 +157,32 @@ public class MapSnapshotterActivity extends AppCompatActivity {
       builder.withImage("Car", Objects.requireNonNull(carBitmap), false)
         .withSources(markerSource)
         .withLayers(markerSymbolLayer);
-      options.withCameraPosition(new CameraPosition.Builder()
-        .target(new LatLng(5.537109374999999,
-          52.07950600379697))
-        .zoom(1)
-        .padding(1, 1, 1, 1)
-        .build()
-      );
+      options
+        .withRegion(null)
+        .withCameraPosition(new CameraPosition.Builder()
+          .target(new LatLng(5.537109374999999,
+            52.07950600379697))
+          .zoom(1)
+          .padding(1, 1, 1, 1)
+          .build()
+        );
     }
 
     options.withStyleBuilder(builder);
     MapSnapshotter snapshotter = new MapSnapshotter(MapSnapshotterActivity.this, options);
+    snapshotter.setObserver(new MapSnapshotter.Observer() {
+      @Override
+      public void onDidFinishLoadingStyle() {
+        Timber.i("onDidFinishLoadingStyle");
+      }
+
+      @Override
+      public void onStyleImageMissing(String imageName) {
+        Bitmap androidIcon = BitmapUtils.getBitmapFromDrawable(getResources().getDrawable(R.drawable.ic_android_2));
+        snapshotter.addImage(imageName, androidIcon, false);
+      }
+    });
+
     snapshotter.start(snapshot -> {
       Timber.i("Got the snapshot");
       ImageView imageView = new ImageView(MapSnapshotterActivity.this);

MapboxGLAndroidSDKTestApp/src/main/java/com/mapbox/mapboxsdk/testapp/activity/turf/MapSnapshotterWithinExpression.kt

@@ -48,6 +48,9 @@ class MapSnapshotterWithinExpression : AppCompatActivity() {
   }
 
   private val snapshotterObserver = object : MapSnapshotter.Observer {
+    override fun onStyleImageMissing(imageName: String?) {
+    }
+
     override fun onDidFinishLoadingStyle() {
       // Show only POI labels inside geometry using within expression
       (snapshotter.getLayer("poi-label") as SymbolLayer).setFilter(