loader_dri3: Honor the request to preserve back buffer content

EGL uses the force_copy parameter to loader_dri3_swap_buffers_msc() to indicate
that it wants to preserve back buffer contents across a buffer swap.

While the loader then turns off server-side page-flipping there's nothing to
guarantee that a new backbuffer isn't chosen when EGL starts to render again,
and that buffer's content is of course undefined.

So rework the functionality:
If the client supports local blits, allow server-side page flipping and when
a new back is grabbed, if needed, blit the old back's content to the new back.
If the client doesn't support local blits, disallow server-side page-flipping
to avoid a client deadlock and then, when grabbing a new back buffer, sleep
until the old back is idle, which may take a substantial time depending on
swap interval.

Signed-off-by: Thomas Hellstrom <[email protected]>
Reviewed-by: Michel Dänzer <[email protected]>

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