Bug 1131446. mFrameHasNoAlpha is mostly pointless. r=seth

Starting with bug 391583 we detect whether a png image has any transparency
during decode. I don't think this is worth doing anymore. We choose the format
of image based on the png header, that should be sufficient for the performance
improvement we desire from opaque images. This gets us faster image decoding.

image/decoders/nsPNGDecoder.cpp

@@ -169,7 +169,6 @@ void nsPNGDecoder::CreateFrame(png_uint_32 x_offset, png_uint_32 y_offset,
   }
 
   mFrameRect = neededRect;
-  mFrameHasNoAlpha = true;
 
   PR_LOG(GetPNGDecoderAccountingLog(), PR_LOG_DEBUG,
          ("PNGDecoderAccounting: nsPNGDecoder::CreateFrame -- created "
@@ -201,7 +200,7 @@ nsPNGDecoder::EndImageFrame()
   mNumFrames++;
 
   Opacity opacity = Opacity::SOME_TRANSPARENCY;
-  if (format == gfx::SurfaceFormat::B8G8R8X8 || mFrameHasNoAlpha) {
+  if (format == gfx::SurfaceFormat::B8G8R8X8) {
     opacity = Opacity::OPAQUE;
   }
 
@@ -739,7 +738,6 @@ nsPNGDecoder::row_callback(png_structp png_ptr, png_bytep new_row,
 
     uint32_t bpr = width * sizeof(uint32_t);
     uint32_t* cptr32 = (uint32_t*)(decoder->mImageData + (row_num*bpr));
-    bool rowHasNoAlpha = true;
 
     if (decoder->mTransform) {
       if (decoder->mCMSLine) {
@@ -788,18 +786,12 @@ nsPNGDecoder::row_callback(png_structp png_ptr, png_bytep new_row,
         if (!decoder->mDisablePremultipliedAlpha) {
           for (uint32_t x=width; x>0; --x) {
             *cptr32++ = gfxPackedPixel(line[3], line[0], line[1], line[2]);
-            if (line[3] != 0xff) {
-              rowHasNoAlpha = false;
-            }
             line += 4;
           }
         } else {
           for (uint32_t x=width; x>0; --x) {
             *cptr32++ = gfxPackedPixelNoPreMultiply(line[3], line[0], line[1],
                                                     line[2]);
-            if (line[3] != 0xff) {
-              rowHasNoAlpha = false;
-            }
             line += 4;
           }
         }
@@ -809,10 +801,6 @@ nsPNGDecoder::row_callback(png_structp png_ptr, png_bytep new_row,
         png_longjmp(decoder->mPNG, 1);
     }
 
-    if (!rowHasNoAlpha) {
-      decoder->mFrameHasNoAlpha = false;
-    }
-
     if (decoder->mNumFrames <= 1) {
       // Only do incremental image display for the first frame
       // XXXbholley - this check should be handled in the superclass

image/decoders/nsPNGDecoder.h

@@ -86,7 +86,6 @@ class nsPNGDecoder : public Decoder
   uint32_t mCMSMode;
 
   uint8_t mChannels;
-  bool mFrameHasNoAlpha;
   bool mFrameIsHidden;
   bool mDisablePremultipliedAlpha;