nine: Implement threadpool

nine: Implement threadpool

DRI_PRIME setups have different issues due the lack of dma-buf fences
support in the drivers. For DRI3 DRI_PRIME, a race can appear, making
tearings visible, or worse showing older content than expected. Until
dma-buf fences are well supported (and by all drivers), an alternative
is to send the buffers to the server only when rendering has finished.
Since waiting the rendering has finished in the main thread has a
performance impact, this patch uses an additional thread to offload the
wait and the sending of the buffers to the server.

Acked-by: Jose Fonseca <jfonseca@vmware.com>
Reviewed-by: David Heidelberg <david@ixit.cz>
Signed-off-by: Axel Davy <axel.davy@ens.fr>