Code review comment for lp:~kdub/mir/server-sent-buffer

Just returning one fence that signals (perhaps we would epoll on it?) still has the problem that we have to go and get the FD that indicates when the buffer is ready. (the 'standard' android fences). The android fences could be (and, without confirming, probably are) a different FD each time a buffer fence is returned from the driver, so we have to send that fd via ipc each time we get one.

There's two timelines going on... one is from the driver, and that fence protects the buffers content (when the GPU/display thinks the buffer is ready). The second timeline is when mir thinks its ready (perhaps there's screenshot going on, or we have to hold the buffer a bit longer for a convoluted multimonitor scenario), and both have to be clear before the gpu can go and write in the buffer on the client side.