[core] Load sources only if there is at least one visible layer

src/mbgl/map/source.cpp

@@ -39,9 +39,7 @@ Source::Source() {}
 Source::~Source() = default;
 
 bool Source::isLoaded() const {
-    if (!loaded) {
-        return false;
-    }
+    if (!loaded) return false;
 
     for (const auto& tile : tiles) {
         if (tile.second->data->getState() != TileData::State::parsed) {
@@ -52,6 +50,10 @@ bool Source::isLoaded() const {
     return true;
 }
 
+bool Source::isLoading() const {
+    return !loaded && req.operator bool();
+}
+
 // Note: This is a separate function that must be called exactly once after creation
 // The reason this isn't part of the constructor is that calling shared_from_this() in
 // the constructor fails.