add src-rect-constraint to drawImageRect

Follow-on work
- unify around SrcRectConstraint (i.e. drawBitmapRect)
- remove silly drawBitmapRectToRect alias
- clean-up (possibly remove) alias problems around drawBitmapRect + IRect parameter

BUG=skia:

Review URL: https://codereview.chromium.org/1228083004
diff --git a/gm/convexpolyclip.cpp b/gm/convexpolyclip.cpp
index 2eabeba..c1df69e 100644
--- a/gm/convexpolyclip.cpp
+++ b/gm/convexpolyclip.cpp
@@ -144,9 +144,7 @@
         SkPaint bgPaint;
         bgPaint.setAlpha(0x15);
         SkISize size = canvas->getDeviceSize();
-        SkRect dstRect = SkRect::MakeWH(SkIntToScalar(size.fWidth),
-                                        SkIntToScalar(size.fHeight));
-        canvas->drawBitmapRectToRect(fBmp, NULL, dstRect, &bgPaint);
+        canvas->drawBitmapRect(fBmp, SkRect::MakeIWH(size.fWidth, size.fHeight), &bgPaint);
 
         static const char kTxt[] = "Clip Me!";
         SkPaint txtPaint;