Don't fail SkMergeImageFilter if one of the inputs are empty

src/effects/SkMergeImageFilter.cpp

 /*
  * Copyright 2012 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkMergeImageFilter.h"
 #include "SkCanvas.h"
 #include "SkDevice.h"
@@ skipping 59 matching lines @@
     const int x0 = bounds.left();
     const int y0 = bounds.top();
 
     SkAutoTUnref<SkBaseDevice> dst(proxy->createDevice(bounds.width(), bounds.height()));
     if (NULL == dst) {
         return false;
     }
     SkCanvas canvas(dst);
     SkPaint paint;
 
+    bool didProduceResult = false;
     int inputCount = countInputs();
     for (int i = 0; i < inputCount; ++i) {
         SkBitmap tmp;
         const SkBitmap* srcPtr;
         SkIPoint pos = SkIPoint::Make(0, 0);
         SkImageFilter* filter = getInput(i);
         if (filter) {
             if (!filter->filterImage(proxy, src, ctx, &tmp, &pos)) {
-                return false;
+                continue;
             }
             srcPtr = &tmp;
         } else {
             srcPtr = &src;
         }
 
         if (fModes) {
             paint.setXfermodeMode((SkXfermode::Mode)fModes[i]);
         } else {
             paint.setXfermode(NULL);
         }
         canvas.drawSprite(*srcPtr, pos.x() - x0, pos.y() - y0, &paint);
+        didProduceResult = true;
     }
 
+    if (!didProduceResult)
+        return false;
+
     offset->fX = bounds.left();
     offset->fY = bounds.top();
     *result = dst->accessBitmap(false);
     return true;
 }
 
 SkFlattenable* SkMergeImageFilter::CreateProc(SkReadBuffer& buffer) {
     Common common;
     if (!common.unflatten(buffer, -1)) {
         return NULL;
@@ skipping 34 matching lines @@
     for (int i = 0; i < this->countInputs(); ++i) {
         SkImageFilter* filter = this->getInput(i);
         str->appendf("%d: (", i);
         filter->toString(str);
         str->appendf(")");
     }
 
     str->append(")");
 }
 #endif