Support decoding opaque to *premul

src/codec/SkJpegCodec.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 "SkCodec.h"
 #include "SkJpegCodec.h"
 #include "SkJpegDecoderMgr.h"
@@ skipping 146 matching lines @@
  * Sets the output color space
  */
 bool SkJpegCodec::setOutputColorSpace(const SkImageInfo& dst) {
     const SkImageInfo& src = this->getInfo();
 
     // Ensure that the profile type is unchanged
     if (dst.profileType() != src.profileType()) {
         return false;
     }
 
-    // Ensure that the alpha type is opaque
+    if (kUnknown_SkAlphaType == dst.alphaType()) {
+        return false;
+    }
+
     if (kOpaque_SkAlphaType != dst.alphaType()) {
-        return false;
+        SkCodecPrintf("Warning: an opaque image should be decoded as opaque "
+                      "- it is being decoded as non-opaque, which will draw slower\n");
     }
 
     // Check if we will decode to CMYK because a conversion to RGBA is not supported
     J_COLOR_SPACE colorSpace = fDecoderMgr->dinfo()->jpeg_color_space;
     bool isCMYK = JCS_CMYK == colorSpace || JCS_YCCK == colorSpace;
 
     // Check for valid color types and set the output color space
     switch (dst.colorType()) {
         case kN32_SkColorType:
             if (isCMYK) {
@@ skipping 457 matching lines @@
 
         JDIMENSION linesRead = jpeg_read_raw_data(dinfo, yuv, numRowsPerBlock);
         if (linesRead < remainingRows) {
             // FIXME: Handle incomplete YUV decodes without signalling an error.
             return kInvalidInput;
         }
     }
 
     return kSuccess;
 }