Implement Canvas.drawPoints

This enables drawing polygons faster than using Paths.

Fixes flutter/flutter#92

sky/engine/core/dart/painting.dart

@@ -433,13 +433,29 @@ class ImageShader extends Shader {
 }
 
 /// Defines how a list of points is interpreted when drawing a set of triangles.
-/// See Skia or OpenGL documentation for more details.
 enum VertexMode {
+  /// Draw each sequence of three points as the vertices of a triangle.
   triangles,
+
+  /// Draw each sliding window of three points as the vertices of a triangle.
   triangleStrip,
+
+  /// Draw the first point and each sliding window of two points as the vertices of a triangle.
   triangleFan,
 }
 
+/// Defines how a list of points is interpreted when drawing a set of points.
+enum PointMode {
+  // Draw each point separately.
+  points,
+
+  // Draw each sequence of two points as a line segment.
+  lines,
+
+  // Draw the entire sequence of point as the vetices of a polygon.
+  polygon,
+}
+
 /// An interface for recording graphical operations.
 ///
 /// [Canvas] objects are used in creating [Picture] objects, which can
@@ -742,6 +758,12 @@ class Canvas extends NativeFieldWrapperClass2 {
   }
   void _drawParagraph(Paragraph paragraph, double x, double y) native "Canvas_drawParagraph";
 
+  /// Draws a sequence of points according to the given [PointMode].
+  void drawPoints(PointMode pointMode, List<Point> points, Paint paint) {
+    _drawPoints(pointMode.index, _encodePointList(points), paint);
+  }
+  void _drawPoints(int pointMode, Float32List points, Paint paint) native "Canvas_drawPoints";
+
   void drawVertices(VertexMode vertexMode,
                     List<Point> vertices,
                     List<Point> textureCoordinates,

sky/engine/core/painting/Canvas.cpp

@@ -53,6 +53,7 @@ IMPLEMENT_WRAPPERTYPEINFO(ui, Canvas);
   V(Canvas, drawImageNine) \
   V(Canvas, drawPicture) \
   V(Canvas, drawParagraph) \
+  V(Canvas, drawPoints) \
   V(Canvas, drawVertices) \
   V(Canvas, drawAtlas)
 
@@ -329,6 +330,22 @@ void Canvas::drawParagraph(Paragraph* paragraph, double x, double y) {
     paragraph->paint(this, x, y);
 }
 
+void Canvas::drawPoints(SkCanvas::PointMode pointMode,
+                        const Float32List& points,
+                        const Paint& paint) {
+  if (!m_canvas)
+    return;
+
+  static_assert(sizeof(SkPoint) == sizeof(float) * 2, "SkPoint doesn't use floats.");
+
+  m_canvas->drawPoints(
+    pointMode,
+    points.num_elements(),
+    reinterpret_cast<const SkPoint*>(points.data()),
+    *paint.paint()
+  );
+}
+
 void Canvas::drawVertices(
     SkCanvas::VertexMode vertexMode,
     const Float32List& vertices,

sky/engine/core/painting/Canvas.h

@@ -22,6 +22,9 @@ class CanvasImage;
 class DartLibraryNatives;
 class Paragraph;
 
+template <>
+struct DartConverter<SkCanvas::PointMode> : public DartConverterInteger<SkCanvas::PointMode> {};
+
 template <>
 struct DartConverter<SkCanvas::VertexMode> : public DartConverterInteger<SkCanvas::VertexMode> {};
 
@@ -101,6 +104,10 @@ class Canvas : public base::RefCountedThreadSafe<Canvas>, public DartWrappable {
     void drawPicture(Picture* picture);
     void drawParagraph(Paragraph* paragraph, double x, double y);
 
+    void drawPoints(SkCanvas::PointMode pointMode,
+                    const Float32List& points,
+                    const Paint& paint);
+
     void drawVertices(SkCanvas::VertexMode vertexMode,
                       const Float32List& vertices,
                       const Float32List& textureCoordinates,