Express (GLSL expression, possibly known value) pairs as a class

src/effects/SkLumaXfermode.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 "SkLumaXfermode.h"
 #include "SkColorPriv.h"
 #include "SkFlattenableBuffers.h"
@@ skipping 195 matching lines @@
                                     const GrDrawEffect& effect,
                                     EffectKey key,
                                     const char* outputColor,
                                     const char* inputColor,
                                     const TextureSamplerArray& samplers) {
 
     const GrLumaMaskEffect& lumaEffect = effect.castEffect<GrLumaMaskEffect>();
     const char* dstColor = builder->dstColor();
     SkASSERT(NULL != dstColor);
     if (NULL == inputColor) {
-        inputColor = GrGLSLOnesVecf(4);
+        inputColor = GrGLSLExpr<4>(1).c_str();
     }
 
     const char *opA = lumaOpA<char>(lumaEffect.getMode(), inputColor, dstColor);
     const char *opB = lumaOpB<char>(lumaEffect.getMode(), inputColor, dstColor);
 
     builder->fsCodeAppendf("\t\tfloat luma = dot(vec3(%f, %f, %f), %s.rgb); \n",
                            SK_ITU_BT709_LUM_COEFF_R,
                            SK_ITU_BT709_LUM_COEFF_G,
                            SK_ITU_BT709_LUM_COEFF_B,
                            opB);
@@ skipping 41 matching lines @@
     // No background texture support.
     if (effect && !background) {
         *effect = GrLumaMaskEffect::Create(fMode);
         return true;
     }
 
     return false;
 }
 
 #endif // SK_SUPPORT_GPU