Add Alpha8 support to SkPNGImageEncoder

src/codec/SkSwizzler.cpp

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkCodecPriv.h"
 #include "SkColorPriv.h"
 #include "SkOpts.h"
@@ skipping 694 matching lines @@
                         }
                         break;
                     case 8:
                         switch (dstInfo.colorType()) {
                             case kRGBA_8888_SkColorType:
                             case kBGRA_8888_SkColorType:
                                 proc = &swizzle_gray_to_n32;
                                 fastProc = &fast_swizzle_gray_to_n32;
                                 break;
                             case kGray_8_SkColorType:
+                            case kAlpha_8_SkColorType:
                                 proc = &sample1;
                                 fastProc = ©
                                 break;
                             case kRGB_565_SkColorType:
                                 proc = &swizzle_gray_to_565;
                                 break;
                             default:
                                 return nullptr;
                         }
                         break;
@@ skipping 311 matching lines @@
     }
 
     return fAllocatedWidth;
 }
 
 void SkSwizzler::swizzle(void* dst, const uint8_t* SK_RESTRICT src) {
     SkASSERT(nullptr != dst && nullptr != src);
     fActualProc(SkTAddOffset<void>(dst, fDstOffsetBytes), src, fSwizzleWidth, fSrcBPP,
             fSampleX * fSrcBPP, fSrcOffsetUnits, fColorTable);
 }