simplify logic in SkPerliNoiseShader for GPU when constant color

src/effects/SkPerlinNoiseShader.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 "SkDither.h"
 #include "SkPerlinNoiseShader.h"
 #include "SkColorFilter.h"
@@ skipping 943 matching lines @@
 
     SkMatrix localMatrix = this->getLocalMatrix();
     if (externalLocalMatrix) {
         localMatrix.preConcat(*externalLocalMatrix);
     }
 
     SkMatrix matrix = viewM;
     matrix.preConcat(localMatrix);
 
     if (0 == fNumOctaves) {
-        SkColor clearColor = 0;
         if (kFractalNoise_Type == fType) {
-            clearColor = SkColorSetARGB(paint.getAlpha() / 2, 127, 127, 127);
-        }
-        SkAutoTUnref<SkColorFilter> cf(SkColorFilter::CreateModeFilter(
-                                                clearColor, SkXfermode::kSrc_Mode));
-        SkTDArray<GrFragmentProcessor*> array;
-        if (cf->asFragmentProcessors(context, &array)) {
-            SkASSERT(1 == array.count());   // modecolorfilter only returns one
-            *fp = array[0];                 // transfer ownership to fp
+            uint32_t alpha = paint.getAlpha() >> 1;
+            uint32_t rgb = alpha >> 1;
+            *paintColor = GrColorPackRGBA(rgb, rgb, rgb, alpha);
+        } else {
+            *paintColor = 0;
         }
         return true;
     }
 
     // Either we don't stitch tiles, either we have a valid tile size
     SkASSERT(!fStitchTiles || !fTileSize.isEmpty());
 
     SkPerlinNoiseShader::PaintingData* paintingData =
             SkNEW_ARGS(PaintingData, (fTileSize, fSeed, fBaseFrequencyX, fBaseFrequencyY, matrix));
     SkAutoTUnref<GrTexture> permutationsTexture(
@@ skipping 54 matching lines @@
     str->append(" seed: ");
     str->appendScalar(fSeed);
     str->append(" stitch tiles: ");
     str->append(fStitchTiles ? "true " : "false ");
 
     this->INHERITED::toString(str);
 
     str->append(")");
 }
 #endif