Make SkComposeImageFilter::onFilterImage filter the bounds given to the inner filter.

src/effects/SkComposeImageFilter.cpp

Index: src/effects/SkComposeImageFilter.cpp
diff --git a/src/effects/SkComposeImageFilter.cpp b/src/effects/SkComposeImageFilter.cpp
index 90844ef9dca23b35189c928846ac7372e945b48b..b06e562baf77c21032644098bb7dc6641b7cfbab 100644
--- a/src/effects/SkComposeImageFilter.cpp
+++ b/src/effects/SkComposeImageFilter.cpp
@@ -23,8 +23,14 @@ void SkComposeImageFilter::computeFastBounds(const SkRect& src, SkRect* dst) con
 
 SkSpecialImage* SkComposeImageFilter::onFilterImage(SkSpecialImage* source, const Context& ctx,
                                                     SkIPoint* offset) const {
+    // The bounds passed to the inner filter must be filtered by the outer
+    // filter, so that the inner filter produces the pixels that the outer
+    // filter requires as input. This matters if the outer filter moves pixels.
+    SkIRect innerClipBounds = ctx.clipBounds();

[Stephen White, 2016/03/21 15:09:18]

I wouldn't bother initializing this, since the call to filterBounds() will
overwrite it anyway.

[jbroman, 2016/03/21 15:13:11]

What I'm unsure about is the case where filterBounds() returns false; AFAICT it
doesn't initialize the value in that case (though other call sites don't seem to
handle this, so I'm a little confused).

Should we allow an uninitialized SkIRect (doesn't even seem to be
zero-initialized) to be used in that case, or should I just not worry about it?

[Stephen White, 2016/03/21 15:18:58]

Yeah, it's kind of a wart on the API: I don't think it's actually possible for
filterBounds() to return false. It only can if onFilterBounds() returns false,
which can only do so if filterBounds() does (recursively). It used to do so for
an SkMatrixImageFilter with a non-invertible matrix, but we just return the
source bounds in that case now. It's an edge case I'm not too worried about. We
should probably remove the boolean at some point.

[jbroman, 2016/03/21 15:22:28]

OK, done.

+    getInput(0)->filterBounds(ctx.clipBounds(), ctx.ctm(), &innerClipBounds);
+    Context innerContext(ctx.ctm(), innerClipBounds, ctx.cache());
     SkIPoint innerOffset = SkIPoint::Make(0, 0);
-    SkAutoTUnref<SkSpecialImage> inner(this->filterInput(1, source, ctx, &innerOffset));
+    SkAutoTUnref<SkSpecialImage> inner(this->filterInput(1, source, innerContext, &innerOffset));
     if (!inner) {
         return nullptr;
     }