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

tests/ImageFilterTest.cpp

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkBitmap.h"
 #include "SkBitmapDevice.h"
 #include "SkBitmapSource.h"
@@ skipping 424 matching lines @@
 
     SkPaint greenPaint;
     greenPaint.setColor(SK_ColorGREEN);
     recordingCanvas->drawRect(SkRect::Make(SkIRect::MakeXYWH(10, 10, 30, 20)), greenPaint);
     SkAutoTUnref<SkPicture> picture(recorder.endRecording());
     SkAutoTUnref<SkImageFilter> pictureFilter(SkPictureImageFilter::Create(picture.get()));
     SkAutoTUnref<SkShader> shader(SkPerlinNoiseShader::CreateTurbulence(SK_Scalar1, SK_Scalar1, 1, 0));
 
     SkAutoTUnref<SkImageFilter> rectShaderFilter(SkRectShaderImageFilter::Create(shader.get()));
 
+    SkAutoTUnref<SkShader> greenColorShader(SkShader::CreateColorShader(SK_ColorGREEN));
+    SkImageFilter::CropRect leftSideCropRect(SkRect::MakeXYWH(0, 0, 32, 64));
+    SkAutoTUnref<SkImageFilter> rectShaderFilterLeft(SkRectShaderImageFilter::Create(greenColorShader.get(), &leftSideCropRect));
+    SkImageFilter::CropRect rightSideCropRect(SkRect::MakeXYWH(32, 0, 32, 64));
+    SkAutoTUnref<SkImageFilter> rectShaderFilterRight(SkRectShaderImageFilter::Create(greenColorShader.get(), &rightSideCropRect));
+
     struct {
         const char*    fName;
         SkImageFilter* fFilter;
     } filters[] = {
         { "color filter", SkColorFilterImageFilter::Create(cf.get()) },
         { "displacement map", SkDisplacementMapEffect::Create(
               SkDisplacementMapEffect::kR_ChannelSelectorType,
               SkDisplacementMapEffect::kB_ChannelSelectorType,
               20.0f, gradient_source.get()) },
         { "blur", SkBlurImageFilter::Create(SK_Scalar1, SK_Scalar1) },
         { "drop shadow", SkDropShadowImageFilter::Create(
               SK_Scalar1, SK_Scalar1, SK_Scalar1, SK_Scalar1, SK_ColorGREEN,
               SkDropShadowImageFilter::kDrawShadowAndForeground_ShadowMode) },
         { "diffuse lighting", SkLightingImageFilter::CreatePointLitDiffuse(
               location, SK_ColorGREEN, 0, 0) },
         { "specular lighting",
               SkLightingImageFilter::CreatePointLitSpecular(location, SK_ColorGREEN, 0, 0, 0) },
         { "matrix convolution",
               SkMatrixConvolutionImageFilter::Create(
                   kernelSize, kernel, gain, bias, SkIPoint::Make(1, 1),
                   SkMatrixConvolutionImageFilter::kRepeat_TileMode, false) },
         { "merge", SkMergeImageFilter::Create(NULL, NULL, SkXfermode::kSrcOver_Mode) },
+        { "merge with disjoint inputs", SkMergeImageFilter::Create(
+              rectShaderFilterLeft, rectShaderFilterRight, SkXfermode::kSrcOver_Mode) },
         { "offset", SkOffsetImageFilter::Create(SK_Scalar1, SK_Scalar1) },
         { "dilate", SkDilateImageFilter::Create(3, 2) },
         { "erode", SkErodeImageFilter::Create(2, 3) },
         { "tile", SkTileImageFilter::Create(SkRect::MakeXYWH(0, 0, 50, 50),
                                             SkRect::MakeXYWH(0, 0, 100, 100), NULL) },
         { "matrix", SkImageFilter::CreateMatrixFilter(matrix, kLow_SkFilterQuality) },
         { "blur and offset", SkOffsetImageFilter::Create(five, five, blur.get()) },
         { "picture and blur", SkBlurImageFilter::Create(five, five, pictureFilter.get()) },
         { "rect shader and blur", SkBlurImageFilter::Create(five, five, rectShaderFilter.get()) },
     };
@@ skipping 707 matching lines @@
         return;
     }
     SkAutoTUnref<SkGpuDevice> device(SkGpuDevice::Create(context,
                                                          SkSurface::kNo_Budgeted,
                                                          SkImageInfo::MakeN32Premul(1, 1),
                                                          0,
                                                          &gProps));
     test_negative_blur_sigma(device, reporter);
 }
 #endif