Support decoding opaque to *premul

src/codec/SkJpegCodec.cpp

Index: src/codec/SkJpegCodec.cpp
diff --git a/src/codec/SkJpegCodec.cpp b/src/codec/SkJpegCodec.cpp
index 50db897a8ee7fb2bc7e26e36407ff93385ddc41c..b834ecbafe5596ead625ffb21b7d8ec7383798f2 100644
--- a/src/codec/SkJpegCodec.cpp
+++ b/src/codec/SkJpegCodec.cpp
@@ -164,11 +164,15 @@ bool SkJpegCodec::setOutputColorSpace(const SkImageInfo& dst) {
         return false;
     }
 
-    // Ensure that the alpha type is opaque
-    if (kOpaque_SkAlphaType != dst.alphaType()) {
+    if (kUnknown_SkAlphaType == dst.alphaType()) {
         return false;
     }
 
+    if (kOpaque_SkAlphaType != dst.alphaType()) {
+        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;