Bug 1064864. Ensure the copying bounds are sane. r=jrmuizel

This prevents things from going really bad on some drivers.

--HG--
extra : rebase_source : c237cf3e6a6174696ebe6d41562a30c8cc0e564d

gfx/layers/d3d11/CompositorD3D11.cpp

@@ -466,7 +466,11 @@ CompositorD3D11::CreateRenderTargetFromSource(const gfx::IntRect &aRect,
     const IntSize& srcSize = sourceD3D11->GetSize();
     MOZ_ASSERT(srcSize.width >= 0 && srcSize.height >= 0,
                "render targets should have nonnegative sizes");
-    if (srcBox.right <= static_cast<uint32_t>(srcSize.width) &&
+    if (srcBox.left >= 0 &&
+        srcBox.top >= 0 &&
+        srcBox.left < srcBox.right &&
+        srcBox.top < srcBox.bottom &&
+        srcBox.right <= static_cast<uint32_t>(srcSize.width) &&
         srcBox.bottom <= static_cast<uint32_t>(srcSize.height)) {
       mContext->CopySubresourceRegion(texture, 0,
                                       0, 0, 0,