Add matrix4 param to Linear gradients

lib/stub_ui/lib/src/ui/painting.dart

@@ -1213,6 +1213,7 @@ abstract class Gradient extends Shader {
     List<Color> colors, [
     List<double> colorStops,
     TileMode tileMode = TileMode.clamp,
+    Float64List matrix4, // TODO(yjbanov): Implement this https://github.com/flutter/flutter/issues/32819
   ]) =>
       _GradientLinear(from, to, colors, colorStops, tileMode);
 

lib/ui/painting.dart

@@ -2722,25 +2722,31 @@ class Gradient extends Shader {
   /// If `from`, `to`, `colors`, or `tileMode` are null, or if `colors` or
   /// `colorStops` contain null values, this constructor will throw a
   /// [NoSuchMethodError].
+  ///
+  /// If `matrix4` is provided, the gradient fill will be transformed by the
+  /// specified 4x4 matrix relative to the local coordinate system. `matrix4` must
+  /// be a column-major matrix packed into a list of 16 values.
   Gradient.linear(
     Offset from,
     Offset to,
     List<Color> colors, [
     List<double> colorStops,
     TileMode tileMode = TileMode.clamp,
+    Float64List matrix4,
   ]) : assert(_offsetIsValid(from)),
        assert(_offsetIsValid(to)),
        assert(colors != null),
        assert(tileMode != null),
+       assert(matrix4 == null || _matrix4IsValid(matrix4)),
        super._() {
     _validateColorStops(colors, colorStops);
     final Float32List endPointsBuffer = _encodeTwoPoints(from, to);
     final Int32List colorsBuffer = _encodeColorList(colors);
     final Float32List colorStopsBuffer = colorStops == null ? null : Float32List.fromList(colorStops);
     _constructor();
-    _initLinear(endPointsBuffer, colorsBuffer, colorStopsBuffer, tileMode.index);
+    _initLinear(endPointsBuffer, colorsBuffer, colorStopsBuffer, tileMode.index, matrix4);
   }
-  void _initLinear(Float32List endPoints, Int32List colors, Float32List colorStops, int tileMode) native 'Gradient_initLinear';
+  void _initLinear(Float32List endPoints, Int32List colors, Float32List colorStops, int tileMode, Float64List matrix4) native 'Gradient_initLinear';
 
   /// Creates a radial gradient centered at `center` that ends at `radius`
   /// distance from the center.

lib/ui/painting/gradient.cc

@@ -42,7 +42,8 @@ fml::RefPtr<CanvasGradient> CanvasGradient::Create() {
 void CanvasGradient::initLinear(const tonic::Float32List& end_points,
                                 const tonic::Int32List& colors,
                                 const tonic::Float32List& color_stops,
-                                SkTileMode tile_mode) {
+                                SkTileMode tile_mode,
+                                const tonic::Float64List& matrix4) {
   FML_DCHECK(end_points.num_elements() == 4);
   FML_DCHECK(colors.num_elements() == color_stops.num_elements() ||
              color_stops.data() == nullptr);
@@ -52,10 +53,16 @@ void CanvasGradient::initLinear(const tonic::Float32List& end_points,
   static_assert(sizeof(SkColor) == sizeof(int32_t),
                 "SkColor doesn't use int32_t.");
 
+  SkMatrix sk_matrix;
+  bool has_matrix = matrix4.data() != nullptr;
+  if (has_matrix) {
+    sk_matrix = ToSkMatrix(matrix4);
+  }
+
   set_shader(UIDartState::CreateGPUObject(SkGradientShader::MakeLinear(
       reinterpret_cast<const SkPoint*>(end_points.data()),
       reinterpret_cast<const SkColor*>(colors.data()), color_stops.data(),
-      colors.num_elements(), tile_mode)));
+      colors.num_elements(), tile_mode, 0, has_matrix ? &sk_matrix : nullptr)));
 }
 
 void CanvasGradient::initRadial(double center_x,

lib/ui/painting/gradient.h

@@ -31,7 +31,8 @@ class CanvasGradient : public Shader {
   void initLinear(const tonic::Float32List& end_points,
                   const tonic::Int32List& colors,
                   const tonic::Float32List& color_stops,
-                  SkTileMode tile_mode);
+                  SkTileMode tile_mode,
+                  const tonic::Float64List& matrix4);
 
   void initRadial(double center_x,
                   double center_y,