Revert of Change quality settings on SkImageDecoder_libjpeg

src/images/SkImageDecoder_libjpeg.cpp

Index: src/images/SkImageDecoder_libjpeg.cpp
diff --git a/src/images/SkImageDecoder_libjpeg.cpp b/src/images/SkImageDecoder_libjpeg.cpp
index 9de17337038d47dfc1b6b2f02605d7eb18de15d7..7216239e80b14b0f6e81dad66bdbfdcc4875a6d2 100644
--- a/src/images/SkImageDecoder_libjpeg.cpp
+++ b/src/images/SkImageDecoder_libjpeg.cpp
@@ -383,11 +383,35 @@
 }
 
 /**
+ *  Common code for turning off upsampling and smoothing. Turning these
+ *  off helps performance without showing noticable differences in the
+ *  resulting bitmap.
+ */
+static void turn_off_visual_optimizations(jpeg_decompress_struct* cinfo) {
+    SkASSERT(cinfo != nullptr);
+    /* this gives about 30% performance improvement. In theory it may
+       reduce the visual quality, in practice I'm not seeing a difference
+     */
+    cinfo->do_fancy_upsampling = 0;
+
+    /* this gives another few percents */
+    cinfo->do_block_smoothing = 0;
+}
+
+/**
  * Common code for setting the dct method.
  */
 static void set_dct_method(const SkImageDecoder& decoder, jpeg_decompress_struct* cinfo) {
     SkASSERT(cinfo != nullptr);
+#ifdef DCT_IFAST_SUPPORTED
+    if (decoder.getPreferQualityOverSpeed()) {
+        cinfo->dct_method = JDCT_ISLOW;
+    } else {
+        cinfo->dct_method = JDCT_IFAST;
+    }
+#else
     cinfo->dct_method = JDCT_ISLOW;
+#endif
 }
 
 SkColorType SkJPEGImageDecoder::getBitmapColorType(jpeg_decompress_struct* cinfo) {
@@ -556,6 +580,8 @@
     SkASSERT(1 == cinfo.scale_num);
     cinfo.scale_denom = sampleSize;
 
+    turn_off_visual_optimizations(&cinfo);
+
     const SkColorType colorType = this->getBitmapColorType(&cinfo);
     const SkAlphaType alphaType = kAlpha_8_SkColorType == colorType ?
                                       kPremul_SkAlphaType : kOpaque_SkAlphaType;
@@ -881,6 +907,8 @@
     SkASSERT(1 == cinfo.scale_num);
     cinfo.scale_denom = 1;
 
+    turn_off_visual_optimizations(&cinfo);
+
 #ifdef ANDROID_RGB
     cinfo.dither_mode = JDITHER_NONE;
 #endif
@@ -954,6 +982,8 @@
     // jpeg_init_read_tile_scanline will check out_color_space again after
     // that change (when it calls jinit_color_deconverter).
     (void) this->getBitmapColorType(cinfo);
+
+    turn_off_visual_optimizations(cinfo);
 
     // instead of jpeg_start_decompress() we start a tiled decompress
     if (!imageIndex->startTileDecompress()) {