fixing onSkipScanlines png error

There was a bug in onSkipScanlines where NULL was being passed instead of the SrcRow to png_read_rows.
This caused SkipScanlines to appear to be skipping, as fCurrScanline was updated and SkipScanlines returned success, however png_read_rows was not actually reading anything, and no lines were skipped.

Review URL: https://codereview.chromium.org/1159853003

1 files changed, 6 insertions, 2 deletions

diff --git a/src/codec/SkCodec_libpng.cpp b/src/codec/SkCodec_libpng.cpp
index 9cd382092e..c2a032e84e 100644
--- a/src/codec/SkCodec_libpng.cpp
+++ b/src/codec/SkCodec_libpng.cpp
@@ -624,8 +624,12 @@ public:
             SkCodecPrintf("setjmp long jump!\n");
             return SkImageGenerator::kInvalidInput;
         }
-
-        png_read_rows(fCodec->fPng_ptr, png_bytepp_NULL, png_bytepp_NULL, count);
+        //there is a potential tradeoff of memory vs speed created by putting this in a loop. 
+        //calling png_read_rows in a loop is insignificantly slower than calling it once with count 
+        //as png_read_rows has it's own loop which calls png_read_row count times.
+        for (int i = 0; i < count; i++) {
+            png_read_rows(fCodec->fPng_ptr, &fSrcRow, png_bytepp_NULL, 1);
+        }
         return SkImageGenerator::kSuccess;
     }