Copied from: https://hg.mozilla.org/releases/mozilla-esr38/rev/9d14787bd10e
Mozilla Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1146335

# HG changeset patch
# User Seth Fowler <mark.seth.fowler@gmail.com>
# Date 1428627143 25200
# Node ID 9d14787bd10e6f3013263a2cae0bcc78bebde1db
# Parent  aaf922ae679685acb5d2b8ffa5f0bf22f1e6987a
Bug 1146335 (Part 1) - Add assertions and fix style issues in image::Downscaler. r=tn a=lizzard

diff --git a/image/src/Downscaler.cpp b/image/src/Downscaler.cpp
--- a/image/src/Downscaler.cpp
+++ b/image/src/Downscaler.cpp
@@ -72,23 +72,25 @@ Downscaler::BeginFrame(const nsIntSize& 
   mOutputBuffer = aOutputBuffer;
   mHasAlpha = aHasAlpha;
 
   ResetForNextProgressivePass();
   ReleaseWindow();
 
   auto resizeMethod = skia::ImageOperations::RESIZE_LANCZOS3;
 
-  skia::resize::ComputeFilters(resizeMethod, mOriginalSize.width,
-                               mTargetSize.width, 0,
-                               mTargetSize.width, mXFilter.get());
+  skia::resize::ComputeFilters(resizeMethod,
+                               mOriginalSize.width, mTargetSize.width,
+                               0, mTargetSize.width,
+                               mXFilter.get());
 
-  skia::resize::ComputeFilters(resizeMethod, mOriginalSize.height,
-                               mTargetSize.height, 0,
-                               mTargetSize.height, mYFilter.get());
+  skia::resize::ComputeFilters(resizeMethod,
+                               mOriginalSize.height, mTargetSize.height,
+                               0, mTargetSize.height,
+                               mYFilter.get());
 
   // Allocate the buffer, which contains scanlines of the original image.
   size_t bufferLen = mOriginalSize.width * sizeof(uint32_t);
   mRowBuffer = MakeUnique<uint8_t[]>(bufferLen);
   if (MOZ_UNLIKELY(!mRowBuffer)) {
     return NS_ERROR_OUT_OF_MEMORY;
   }
 
@@ -126,39 +128,54 @@ void
 Downscaler::ResetForNextProgressivePass()
 {
   mPrevInvalidatedLine = 0;
   mCurrentOutLine = 0;
   mCurrentInLine = 0;
   mLinesInBuffer = 0;
 }
 
+static void
+GetFilterOffsetAndLength(UniquePtr<skia::ConvolutionFilter1D>& aFilter,
+                         int32_t aOutputImagePosition,
+                         int32_t* aFilterOffsetOut,
+                         int32_t* aFilterLengthOut)
+{
+  MOZ_ASSERT(aOutputImagePosition < aFilter->num_values());
+  aFilter->FilterForValue(aOutputImagePosition,
+                          aFilterOffsetOut,
+                          aFilterLengthOut);
+}
+
 void
 Downscaler::CommitRow()
 {
   MOZ_ASSERT(mOutputBuffer, "Should have a current frame");
   MOZ_ASSERT(mCurrentInLine < mOriginalSize.height, "Past end of input");
   MOZ_ASSERT(mCurrentOutLine < mTargetSize.height, "Past end of output");
 
   int32_t filterOffset = 0;
   int32_t filterLength = 0;
-  mYFilter->FilterForValue(mCurrentOutLine, &filterOffset, &filterLength);
+  GetFilterOffsetAndLength(mYFilter, mCurrentOutLine,
+                           &filterOffset, &filterLength);
 
   int32_t inLineToRead = filterOffset + mLinesInBuffer;
   MOZ_ASSERT(mCurrentInLine <= inLineToRead, "Reading past end of input");
   if (mCurrentInLine == inLineToRead) {
     skia::ConvolveHorizontally(mRowBuffer.get(), *mXFilter,
                                mWindow[mLinesInBuffer++], mHasAlpha,
                                /* use_sse2 = */ true);
   }
 
   while (mLinesInBuffer == filterLength &&
          mCurrentOutLine < mTargetSize.height) {
     DownscaleInputLine();
-    mYFilter->FilterForValue(mCurrentOutLine, &filterOffset, &filterLength);
+
+    GetFilterOffsetAndLength(mYFilter, mCurrentOutLine,
+                             &filterOffset, &filterLength);
   }
 
   mCurrentInLine += 1;
 }
 
 bool
 Downscaler::HasInvalidation() const
 {
@@ -184,16 +201,17 @@ Downscaler::DownscaleInputLine()
 {
   typedef skia::ConvolutionFilter1D::Fixed FilterValue;
 
   MOZ_ASSERT(mOutputBuffer);
   MOZ_ASSERT(mCurrentOutLine < mTargetSize.height, "Writing past end of output");
 
   int32_t filterOffset = 0;
   int32_t filterLength = 0;
+  MOZ_ASSERT(mCurrentOutLine < mYFilter->num_values());
   auto filterValues =
     mYFilter->FilterForValue(mCurrentOutLine, &filterOffset, &filterLength);
 
   uint8_t* outputLine =
     &mOutputBuffer[mCurrentOutLine * mTargetSize.width * sizeof(uint32_t)];
   skia::ConvolveVertically(static_cast<const FilterValue*>(filterValues),
                            filterLength, mWindow.get(), mXFilter->num_values(),
                            outputLine, mHasAlpha, /* use_sse2 = */ true);
@@ -202,17 +220,18 @@ Downscaler::DownscaleInputLine()
 
   if (mCurrentOutLine == mTargetSize.height) {
     // We're done.
     return;
   }
 
   int32_t newFilterOffset = 0;
   int32_t newFilterLength = 0;
-  mYFilter->FilterForValue(mCurrentOutLine, &newFilterOffset, &newFilterLength);
+  GetFilterOffsetAndLength(mYFilter, mCurrentOutLine,
+                           &newFilterOffset, &newFilterLength);
 
   int diff = newFilterOffset - filterOffset;
   MOZ_ASSERT(diff >= 0, "Moving backwards in the filter?");
 
   // Shift the buffer. We're just moving pointers here, so this is cheap.
   mLinesInBuffer -= diff;
   mLinesInBuffer = max(mLinesInBuffer, 0);
   for (int32_t i = 0; i < mLinesInBuffer; ++i) {