WINTERMUTE: Avoid using Graphics::copyFrom to copy FMV-frames.

copyFrom frees and reallocates the surface for every update, as long
as the dimensions and format stay the same, we can do with just a
memcpy.

This gives a tiny improvement in the update-part of the Theora-player
(on the order of a bit more than 1 second saved total in the 1:28 long
J.U.L.I.A.-intro)