use forward matrix to determine if we can ignore scale part of a matrix

gm/bitmaprecttest.cpp

Index: gm/bitmaprecttest.cpp
===================================================================
--- gm/bitmaprecttest.cpp	(revision 0)
+++ gm/bitmaprecttest.cpp	(revision 0)
@@ -0,0 +1,67 @@
+/*
+ * Copyright 2013 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "gm.h"
+#include "SkCanvas.h"
+
+static void make_bm(SkBitmap* bm) {
+    bm->setConfig(SkBitmap::kARGB_8888_Config, 60, 60);
+    bm->allocPixels();
+    bm->eraseColor(0);
+
+    SkCanvas canvas(*bm);
+    SkPaint paint;
+    paint.setStyle(SkPaint::kStroke_Style);
+    canvas.drawRect(SkRect::MakeLTRB(0.5f, 0.5f, 29.5f, 29.5f), paint);
+}
+
+// This creates a close, but imperfect concatenation of
+//      scaling the image up by its dst-rect
+//      scaling the image down by the matrix' scale
+//  The bug was that for cases like this, we were incorrectly trying to take a
+//  fast-path in the bitmapshader, but ended up drawing the last col of pixels
+//  twice. The fix resulted in (a) not taking the fast-path, but (b) drawing
+//  the image correctly.
+//
+static void test_bitmaprect(SkCanvas* canvas) {
+    SkBitmap bm;
+    make_bm(&bm);
+
+    canvas->drawBitmap(bm, 150, 45, NULL);
+    
+    SkScalar scale = 0.452560018f;
+    canvas->scale(scale, scale);
+    canvas->drawBitmapRectToRect(bm, NULL, SkRect::MakeXYWH(100, 100, 128, 128), NULL);
+}
+
+class BitmapRectTestGM : public skiagm::GM {
+public:
+    BitmapRectTestGM() {
+
+    }
+
+protected:
+    virtual SkString onShortName() {
+        return SkString("bitmaprectest");
+    }
+
+    virtual SkISize onISize() {
+        return SkISize::Make(320, 240);
+    }
+
+    virtual void onDraw(SkCanvas* canvas) SK_OVERRIDE {
+        test_bitmaprect(canvas);
+    }
+
+private:
+    typedef skiagm::GM INHERITED;
+};
+
+//////////////////////////////////////////////////////////////////////////////
+
+DEF_GM( return new BitmapRectTestGM; )
+