Bug 1556713 - Transform dirtyRect down to <image> r=longsonr

aDirtyRect in PaintSVG() is in outer <svg> coordinate. Image painting
API needs dirty rect in image coordinate. We need to transform it.

Differential Revision: https://phabricator.services.mozilla.com/D33813

--HG--
extra : moz-landing-system : lando

layout/reftests/svg/mask-image-filter-transform-ref.html

@@ -0,0 +1,7 @@
+<svg width="400" height="400">
+  <mask id="m">
+    <image x="20" y="20" width="100" height="100"
+           href="data:image/svg+xml,<svg width='100' height='100' xmlns='http://www.w3.org/2000/svg'><rect width='100%' height='100%' fill='red' /></svg>" />
+  </mask>
+  <rect mask="url(#m)" x="50" y="50" width="50%" height="50%" fill="black"></rect>
+</svg>

layout/reftests/svg/mask-image-filter-transform.html

@@ -0,0 +1,9 @@
+<svg width="400" height="400" viewBox="0 0 800 800">
+  <mask id="m">
+    <g transform="scale(.5,.5)">
+      <image filter="blur(0)" x="80" y="80" width="400" height="400"
+             href="data:image/svg+xml,<svg width='100' height='100' xmlns='http://www.w3.org/2000/svg'><rect width='100%' height='100%' fill='red' /></svg>" />
+    </g>
+  </mask>
+  <rect mask="url(#m)" x="100" y="100" width="50%" height="50%" fill="black"></rect>
+</svg>

layout/reftests/svg/reftest.list

@@ -58,6 +58,7 @@ skip == blend-difference-stacking.html blend-difference-stacking-ref.html # bug
 fuzzy(0-11,0-7155) == blur-inside-clipPath.svg blur-inside-clipPath-ref.svg
 
 == border-radius-01.html pass.svg
+== mask-image-filter-transform.html mask-image-filter-transform-ref.html
 
 == clip-01.svg pass.svg
 == clip-02a.svg clip-02-ref.svg

layout/svg/nsSVGImageFrame.cpp

@@ -307,10 +307,31 @@ void nsSVGImageFrame::PaintSVG(gfxContext& aContext,
                        (mState & NS_FRAME_IS_NONDISPLAY),
                    "Display lists handle dirty rect intersection test");
       dirtyRect = ToAppUnits(*aDirtyRect, appUnitsPerDevPx);
-      // Adjust dirtyRect to match our local coordinate system.
-      nsRect rootRect = nsSVGUtils::TransformFrameRectToOuterSVG(
-          mRect, aTransform, PresContext());
-      dirtyRect.MoveBy(-rootRect.TopLeft());
+
+      // dirtyRect is relative to the outer <svg>, we should transform it
+      // down to <image>.
+      Rect dir(dirtyRect.x, dirtyRect.y, dirtyRect.width, dirtyRect.height);
+      dir.Scale(1.f / AppUnitsPerCSSPixel());
+
+      // FIXME: This isn't correct if there is an inner <svg> enclosing
+      // the <image>. But that seems to be a quite obscure usecase, we can
+      // add a dedicated utility for that purpose to replace the GetCTM
+      // here if necessary.
+      auto mat = SVGContentUtils::GetCTM(
+          static_cast<SVGImageElement*>(GetContent()), false);
+      if (mat.IsSingular()) {
+        return;
+      }
+
+      mat.Invert();
+      dir = mat.TransformRect(dir);
+
+      // x, y offset of <image> is not included in CTM.
+      dir.MoveBy(-x, -y);
+
+      dir.Scale(AppUnitsPerCSSPixel());
+      dir.Round();
+      dirtyRect = nsRect(dir.x, dir.y, dir.width, dir.height);
     }
 
     uint32_t flags = aImgParams.imageFlags;