make alpha read back of bgra/rgba work

src/gpu/GrProgramDesc.h

Index: src/gpu/GrProgramDesc.h
diff --git a/src/gpu/GrProgramDesc.h b/src/gpu/GrProgramDesc.h
index 13dd14935d8dff2992dcfebbeb94ab6a7f1bd0f4..863a318757f23f1016e6ea23fef27f3ce203c35c 100644
--- a/src/gpu/GrProgramDesc.h
+++ b/src/gpu/GrProgramDesc.h
@@ -70,6 +70,9 @@ public:
     }
 
     struct KeyHeader {
+        // Used to handle swizzles because of mismatch between GrPixelConfig of the dst and its
+        // actual format.
+        uint8_t                     fSwapDstRedAndAlpha;
         uint8_t                     fFragPosKey;   // set by GrGLShaderBuilder if there are
                                                    // effects that read the fragment position.
                                                    // Otherwise, 0.
@@ -78,7 +81,6 @@ public:
         int8_t                      fCoverageEffectCnt;
         uint8_t                     fIgnoresCoverage;
     };
-    GR_STATIC_ASSERT(sizeof(KeyHeader) == 5);
 
     int numColorEffects() const {
         return this->header().fColorEffectCnt;