Add pushOffset to SceneBuilder (#6349)

For https://github.com/flutter/flutter/pull/21619#discussion_r220390088

lib/ui/compositing.dart

@@ -64,6 +64,13 @@ class SceneBuilder extends NativeFieldWrapperClass2 {
   }
   void _pushTransform(Float64List matrix4) native 'SceneBuilder_pushTransform';
 
+  /// Pushes an offset operation onto the operation stack.
+  ///
+  /// This is equivalent to [pushTransform] with a matrix with only translation.
+  ///
+  /// See [pop] for details about the operation stack.
+  void pushOffset(double dx, double dy) native 'SceneBuilder_pushOffset';
+
   /// Pushes a rectangular clip operation onto the operation stack.
   ///
   /// Rasterization outside the given rectangle is discarded.

lib/ui/compositing/scene_builder.cc

@@ -93,6 +93,13 @@ void SceneBuilder::pushTransform(const tonic::Float64List& matrix4) {
   PushLayer(std::move(layer), cullRect);
 }
 
+void SceneBuilder::pushOffset(double dx, double dy) {
+  SkMatrix sk_matrix = SkMatrix::MakeTrans(dx, dy);
+  auto layer = std::make_unique<flow::TransformLayer>();
+  layer->set_transform(sk_matrix);
+  PushLayer(std::move(layer), cull_rects_.top().makeOffset(-dx, -dy));
+}
+
 void SceneBuilder::pushClipRect(double left,
                                 double right,
                                 double top,

lib/ui/compositing/scene_builder.h

@@ -33,6 +33,7 @@ class SceneBuilder : public RefCountedDartWrappable<SceneBuilder> {
   ~SceneBuilder() override;
 
   void pushTransform(const tonic::Float64List& matrix4);
+  void pushOffset(double dx, double dy);
   void pushClipRect(double left,
                     double right,
                     double top,