Migrate from MathUtil::inverse() to gfx::Transform::GetInverse()

cc/direct_renderer.cc

Index: cc/direct_renderer.cc
diff --git a/cc/direct_renderer.cc b/cc/direct_renderer.cc
index 70522f2fc0289f7295edba5ffb246e26db4cbe87..e96cc4174088a719e6a9e3f4b372c56207a72629 100644
--- a/cc/direct_renderer.cc
+++ b/cc/direct_renderer.cc
@@ -185,9 +185,12 @@ gfx::RectF DirectRenderer::computeScissorRectForRenderPass(const DrawingFrame& f
     if (frame.rootDamageRect == frame.rootRenderPass->output_rect)
         return renderPassScissor;
 
-    gfx::Transform inverseTransform = MathUtil::inverse(frame.currentRenderPass->transform_to_root_target);
-    gfx::RectF damageRectInRenderPassSpace = MathUtil::projectClippedRect(inverseTransform, frame.rootDamageRect);
-    renderPassScissor.Intersect(damageRectInRenderPassSpace);
+    gfx::Transform inverseTransform(gfx::Transform::kSkipInitialization);
+    if (frame.currentRenderPass->transform_to_root_target.GetInverse(&inverseTransform)) {

[danakj, 2013/01/02 14:45:16]

I'm still not a fan that after the if() statement, if it was false, you have an
inverseTransform variable that is uninitialized data.

I don't see why setting the transform to identity in GetInverse() when it is
going to return false makes this patch any harder to merge back to M25?

Also, I don't expect it would have any impact on perftest at all?

[shawnsingh, 2013/01/02 18:36:01]

The performance improvement comes from avoiding duplicate initialization,
mostly, with kSkipInitialization.  It possibly also comes from initializing by
reference arg rather than return value, but last I checked RVO was working fine
at least on linux.

Yes, I suppose we can initialize-to-identity in the gfx::Transform::GetInverse()
function itself.  I didn't realize that's what you had intended.   

The "harder to merge" part I was concerned about is the changing of semantics to
properly handle all the cases that we are currently not handling correctly - we
should simply never be using the transform if GetInverse returns false.  This
patch doesn't address that issue, and I don't think it should =)

+        // Only intersect inverse-projected damage if the transform is invertible.
+        gfx::RectF damageRectInRenderPassSpace = MathUtil::projectClippedRect(inverseTransform, frame.rootDamageRect);
+        renderPassScissor.Intersect(damageRectInRenderPassSpace);
+    }
 
     return renderPassScissor;
 }