[core] GeometryTile and TileLoader keep strong reference to FileSource

Thus we fix a potential bug: if `Renderer` outlives the `Map` it will hold a stale reference to the `FileSource` instance.

test/tile/vector_tile.test.cpp

@@ -22,10 +22,10 @@ using namespace mbgl;
 
 class VectorTileTest {
 public:
-    FakeFileSource fileSource;
+    std::shared_ptr<FileSource> fileSource = std::make_shared<FakeFileSource>();
     TransformState transformState;
     util::RunLoop loop;
-    style::Style style { fileSource, 1 };
+    style::Style style { *fileSource, 1 };
     AnnotationManager annotationManager { style };
     ImageManager imageManager;
     GlyphManager glyphManager;