wined3d: don't update the screen when gdi back buffers are unlocked

[Vincent Povirk]

This should fix bug 15789.

Stefan tells me that we're supposed to update the screen when the front buffer is unlocked, as that means it could have changed. Instead, we do it when any buffer in the swapchain is unlocked. This makes for a lot of unnecessary copies.

He also told me to make this change to fix it. I don't really understand the surrounding code.

And he tells me that he's too busy to send the change in, and I should do it if I want it to happen soon.

Vincent Povirk
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