The mouseRect is the limit of the mouse position to keep the window between
its minimum and maximum sizes.  The computation was correct if the mouse
started precisely on the window border.  But if the mouse is not exactly at
the border, then the mouse limit has to account for that, because the window
will be resized based on the distance from the starting point.

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.