add one-channel output support to webp encoder

src/images/SkImageDecoder_libwebp.cpp

 /*
  * Copyright 2010, The Android Open Source Project
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
  * You may obtain a copy of the License at
  *
  *     http://www.apache.org/licenses/LICENSE-2.0
  *
  * Unless required by applicable law or agreed to in writing, software
@@ skipping 529 matching lines @@
   const uint8_t* SK_RESTRICT src = (const uint8_t*)in;
   for (int i = 0; i < width; ++i) {
       const uint32_t c = ctable[*src++];
       rgb[0] = SkGetPackedR32(c);
       rgb[1] = SkGetPackedG32(c);
       rgb[2] = SkGetPackedB32(c);
       rgb += 3;
   }
 }
 
+static void Alpha8_To_RGB(const uint8_t* in, uint8_t* rgb, int width,
+                          const SkPMColor* SK_RESTRICT ctable) {
+  const uint8_t* SK_RESTRICT src = (const uint8_t*)in;
+  for (int i = 0; i < width; ++i) {
+      rgb[0] = rgb[1] = rgb[2] = *src++;
+      rgb += 3;
+  }
+}
+
 static ScanlineImporter ChooseImporter(const SkBitmap::Config& config,
                                        bool  hasAlpha,
                                        int*  bpp) {
     switch (config) {
         case SkBitmap::kARGB_8888_Config:
             if (hasAlpha) {
                 *bpp = 4;
                 return ARGB_8888_To_RGBA;
             } else {
                 *bpp = 3;
                 return ARGB_8888_To_RGB;
             }
         case SkBitmap::kARGB_4444_Config:
             if (hasAlpha) {
                 *bpp = 4;
                 return ARGB_4444_To_RGBA;
             } else {
                 *bpp = 3;
                 return ARGB_4444_To_RGB;
             }
         case SkBitmap::kRGB_565_Config:
             *bpp = 3;
             return RGB_565_To_RGB;
         case SkBitmap::kIndex8_Config:
             *bpp = 3;
             return Index8_To_RGB;
+        case SkBitmap::kA8_Config:
+            *bpp = 3;
+            return Alpha8_To_RGB;
         default:
             return NULL;
     }
 }
 
 static int stream_writer(const uint8_t* data, size_t data_size,
                          const WebPPicture* const picture) {
   SkWStream* const stream = (SkWStream*)picture->custom_ptr;
   return stream->write(data, data_size) ? 1 : 0;
 }
@@ skipping 88 matching lines @@
     return SkImageDecoder::kUnknown_Format;
 }
 
 static SkImageEncoder* sk_libwebp_efactory(SkImageEncoder::Type t) {
       return (SkImageEncoder::kWEBP_Type == t) ? SkNEW(SkWEBPImageEncoder) : NULL;
 }
 
 static SkImageDecoder_DecodeReg gDReg(sk_libwebp_dfactory);
 static SkImageDecoder_FormatReg gFormatReg(get_format_webp);
 static SkImageEncoder_EncodeReg gEReg(sk_libwebp_efactory);