Reduce overdraw in MultiPictureDraw Sierpinski GM

As written, all the layers in the MPD Sierpinski GM are full screen. This CL bounds each layer to reduce overdraw.

This CL will cause differences in:

multipicturedraw_sierpinski_simple
multipicturedraw_sierpinski_tiled

but they are/will be suppressed in: (Don't turn on dither for saveLayers which have no paint - https://codereview.chromium.org/619363002/)

Review URL: https://codereview.chromium.org/582633003

gm/multipicturedraw.cpp

@@ -155,6 +155,9 @@ static const SkPicture* make_tri_picture() {
 
     SkCanvas* canvas = recorder.beginRecording(SkIntToScalar(kPicWidth),
                                                SkIntToScalar(kPicHeight));
+    SkRect r = tri.getBounds();
+    r.outset(2.0f, 2.0f);       // outset for stroke
+    canvas->clipRect(r);
     // The saveLayer/restore block is to exercise layer hoisting
     canvas->saveLayer(NULL, NULL);
         canvas->drawPath(tri, fill);