The wined3d front buffer is not a render target.

Note that it still isn't necessarily safe to render in this case, because
there are places where we assume the first render target is always non-NULL in
order to determine e.g. framebuffer dimensions. It's now the responsibility of
the caller to ensure that doesn't happen.

wined3d: Properly check if the swapchain format is WINED3DFMT_P8_UINT in primary_render_target_is_p8().

The surface needs to be part of a swapchain to be flipped, but that doesn't
have much to do with WINED3DUSAGE_RENDERTARGET. We can just remove the check
here, because ddraw_surface7_Flip() already has the correct check.

r500 does not support anything but AMD's extension depth texture formats
on Windows 7. Plain surfaces work OK with the builtin formats.