Add capture snapshot as data to SkWriter32, use it to optimise record->playback.

src/core/SkWriter32.cpp

Index: src/core/SkWriter32.cpp
diff --git a/src/core/SkWriter32.cpp b/src/core/SkWriter32.cpp
index fe33e4a39cf011d1ce54d1d9a08c321d99f707fe..c7bfd92d56aafd832868c42f2690f3bcd8082ef3 100644
--- a/src/core/SkWriter32.cpp
+++ b/src/core/SkWriter32.cpp
@@ -74,4 +74,32 @@ void SkWriter32::growToAtLeast(size_t size) {
         // we were external, so copy in the data
         memcpy(fData, fExternal, fUsed);
     }
+    // Invalidate the snapshot, we know it is no longer useful.
+    fSnapshot.reset(NULL);
+}
+
+SkData* SkWriter32::snapshotAsData() const {
+    // get a non const version of this, we are only conceptually const
+    SkWriter32& mutable_this = *const_cast<SkWriter32*>(this);
+    // we use size change detection to invalidate the cached data
+    if ((fSnapshot.get() != NULL) && (fSnapshot->size() != fUsed)) {
+        mutable_this.fSnapshot.reset(NULL);
+    }
+    if (fSnapshot.get() == NULL) {
+        uint8_t* buffer = NULL;
+        if ((fExternal != NULL) && (fData == fExternal)) {
+            // We need to copy to an allocated buffer before returning.
+            buffer = (uint8_t*)sk_malloc_throw(fUsed);
+            memcpy(buffer, fData, fUsed);
+        } else {
+            buffer = mutable_this.fInternal.detach();
+            // prepare us to do copy on write, by pretending the data buffer
+            // is external and size limited
+            mutable_this.fData = buffer;
+            mutable_this.fCapacity = fUsed;
+            mutable_this.fExternal = buffer;
+        }
+        mutable_this.fSnapshot.reset(SkData::NewFromMalloc(buffer, fUsed));
+    }
+    return SkRef(fSnapshot.get()); // Take an extra ref for the caller.
 }