[core] clear source when removed from style

src/mbgl/style/source_impl.cpp

@@ -44,6 +44,10 @@ bool Source::Impl::isLoaded() const {
 
     return true;
 }
+
+void Source::Impl::detach() {
+    invalidateTiles();
+}
 
 void Source::Impl::invalidateTiles() {
     tiles.clear();

src/mbgl/style/source_impl.hpp

@@ -87,6 +87,9 @@ class Source::Impl : public TileObserver, private util::noncopyable {
     // be initialized to true so that Style::isLoaded() does not produce false positives if
     // called before Style::recalculate().
     bool enabled = true;
+
+    // Detaches from the style
+    void detach();
 
 protected:
     void invalidateTiles();

src/mbgl/style/style.cpp

@@ -166,6 +166,7 @@ std::unique_ptr<Source> Style::removeSource(const std::string& id) {
     sources.erase(it);
     updateBatch.sourceIDs.erase(id);
 
+    source->baseImpl->detach();
     return source;
 }