There seemed to be some duplication of code in the UpdateSurface method,
as well as an issue with updating that caused some reftests to fail.
In this patch, I remove the mSurface member variable from
CanvasLayerD3D9.h, and instead call mDrawTarget->Snapshot() as needed.
I also reuse the code to draw into the D3DLOCKED_RECT so that we don't
duplicate that portion anymore. It seems now to be more similar to the way
CanvasLayerD3D10 works.
The reftests now pass.