[core] Add interface to add sprites to the style

Unlike annotations, they will go away when we change the style.

include/mbgl/map/map.hpp

@@ -161,6 +161,10 @@ public:
     void addLayer(std::unique_ptr<style::Layer>, const optional<std::string>& beforeLayerID = {});
     void removeLayer(const std::string& layerID);
 
+    // Add image, bound to the style
+    void addImage(const std::string&, std::unique_ptr<const SpriteImage>);
+    void removeImage(const std::string&);
+
     // Defaults
     std::string getStyleName() const;
     LatLng getDefaultLatLng() const;

src/mbgl/map/map.cpp

@@ -823,6 +823,30 @@ void Map::removeLayer(const std::string& id) {
     impl->view.deactivate();
 }
 
+void Map::addImage(const std::string& name, std::unique_ptr<const SpriteImage> image) {
+    if (!impl->style) {
+        return;
+    }
+
+    impl->styleMutated = true;
+    impl->style->spriteAtlas->setSprite(name, std::move(image));
+    impl->style->spriteAtlas->updateDirty();
+
+    update(Update::Repaint);
+}
+
+void Map::removeImage(const std::string& name) {
+    if (!impl->style) {
+        return;
+    }
+
+    impl->styleMutated = true;
+    impl->style->spriteAtlas->removeSprite(name);
+    impl->style->spriteAtlas->updateDirty();
+
+    update(Update::Repaint);
+}
+
 #pragma mark - Defaults
 
 std::string Map::getStyleName() const {