Make converting from 4x4 to 3x3 explicit

Change-Id: I4d190d6831cb517d5e9f22b3f872013367bddc08
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/264856
Reviewed-by: Robert Phillips <robertphillips@google.com>
diff --git a/samplecode/Sample3D.cpp b/samplecode/Sample3D.cpp
index df3fab1..1410fb6 100644
--- a/samplecode/Sample3D.cpp
+++ b/samplecode/Sample3D.cpp
@@ -227,7 +227,7 @@
         SkMatrix44 trans;
         trans.setTranslate(200, 200, 0);   // center of the rotation
 
-        canvas->concat(trans * fRot * m * inv(trans));
+        canvas->experimental_concat44(trans * fRot * m * inv(trans));
 
         if (!front(canvas->experimental_getLocalToDevice())) {
             return;