Do some minor pre cleanup work before converting all xfermodes to XPs.

src/effects/SkXfermodeImageFilter.cpp

 /*
  * Copyright 2013 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 "SkXfermodeImageFilter.h"
 #include "SkCanvas.h"
 #include "SkDevice.h"
@@ skipping 94 matching lines @@
     if (fMode) {
         fMode->toString(str);
     }
     str->append("))");
 }
 #endif
 
 #if SK_SUPPORT_GPU
 
 bool SkXfermodeImageFilter::canFilterImageGPU() const {
+    // This is checked with the background texture as NULL here since we have no actual texture,

[bsalomon, 2015/01/14 15:18:13]

Does this comment add much? Maybe just "Call asFragmentProcessor() with NULLs to
use it as a query without creating a processor."

+    // however it will be assumed that a valid texture is used if asFragmentProcessor is called
+    // outside of checking return values.
     return fMode && fMode->asFragmentProcessor(NULL, NULL) && !cropRectIsSet();
 }
 
 bool SkXfermodeImageFilter::filterImageGPU(Proxy* proxy,
                                            const SkBitmap& src,
                                            const Context& ctx,
                                            SkBitmap* result,
                                            SkIPoint* offset) const {
     SkBitmap background = src;
     SkIPoint backgroundOffset = SkIPoint::Make(0, 0);
     if (getInput(0) && !getInput(0)->getInputResultGPU(proxy, src, ctx, &background,
                                                        &backgroundOffset)) {
         return onFilterImage(proxy, src, ctx, result, offset);
     }
     GrTexture* backgroundTex = background.getTexture();
+
+    if (NULL == backgroundTex) {
+        SkASSERT(false);
+        return false;
+    }
+
     SkBitmap foreground = src;
     SkIPoint foregroundOffset = SkIPoint::Make(0, 0);
     if (getInput(1) && !getInput(1)->getInputResultGPU(proxy, src, ctx, &foreground,
                                                        &foregroundOffset)) {
         return onFilterImage(proxy, src, ctx, result, offset);
     }
     GrTexture* foregroundTex = foreground.getTexture();
     GrContext* context = foregroundTex->getContext();
 
     GrFragmentProcessor* xferProcessor = NULL;
@@ skipping 30 matching lines @@
     context->drawRect(paint, SkMatrix::I(), srcRect);
 
     offset->fX = backgroundOffset.fX;
     offset->fY = backgroundOffset.fY;
     WrapTexture(dst, src.width(), src.height(), result);
     return true;
 }
 
 #endif