Add a Via to DM that records into two pictures and draws using the second.

dm/DM.cpp

Index: dm/DM.cpp
diff --git a/dm/DM.cpp b/dm/DM.cpp
index 3ad82262c4a17d01240dd677931430b0020844dd..5d85e22b3a8e989d30ff358ead3f1098df882c37 100644
--- a/dm/DM.cpp
+++ b/dm/DM.cpp
@@ -320,6 +320,7 @@ static Sink* create_via(const char* tag, Sink* wrapped) {
 #define VIA(t, via, ...) if (0 == strcmp(t, tag)) { return new via(__VA_ARGS__); }
     VIA("pipe",      ViaPipe,          wrapped);
     VIA("serialize", ViaSerialization, wrapped);
+    VIA("2ndpic",    ViaSecondPicture, wrapped);
     VIA("tiles",     ViaTiles, 256, 256,               NULL, wrapped);
     VIA("tiles_rt",  ViaTiles, 256, 256, new SkRTreeFactory, wrapped);