Track scissored damage area for WebGL

Source/core/html/canvas/WebGLRenderingContextBase.cpp

Index: Source/core/html/canvas/WebGLRenderingContextBase.cpp
diff --git a/Source/core/html/canvas/WebGLRenderingContextBase.cpp b/Source/core/html/canvas/WebGLRenderingContextBase.cpp
index 83001cc340df67394833985a36c25f2d597952f9..1aec211c2b81e34da7edaf82c2079271c0038493 100644
--- a/Source/core/html/canvas/WebGLRenderingContextBase.cpp
+++ b/Source/core/html/canvas/WebGLRenderingContextBase.cpp
@@ -802,7 +802,10 @@ void WebGLRenderingContextBase::markContextChanged(ContentChangeType changeType)
     if (renderBox && renderBox->hasAcceleratedCompositing()) {
         m_markedCanvasDirty = true;
         canvas()->clearCopiedImage();
-        renderBox->contentChanged(changeType);
+        IntRect rect = IntRect(IntPoint(0, 0), clampedCanvasSize());
+        if (!m_flippedScissorBox.isEmpty())
+            rect.intersect(m_flippedScissorBox);
+        renderBox->contentChanged(changeType, rect);
     } else {
         if (!m_markedCanvasDirty) {
             m_markedCanvasDirty = true;
@@ -3239,6 +3242,8 @@ void WebGLRenderingContextBase::scissor(GLint x, GLint y, GLsizei width, GLsizei
         return;
     if (!validateSize("scissor", width, height))
         return;
+
+    m_flippedScissorBox = IntRect(x, canvas()->height() - height, width, height - y);
     webContext()->scissor(x, y, width, height);
 }