This adds a flag, -p,--presentation-timings, that enables input lag
measuring using the presentation time Wayland protocol.
When enabled, we store a timestamp when we *send* a key to the
slave. Then, when we commit a frame for rendering to the compositor,
we request presentation feedback. We also store a timestamp for when
the frame was committed.
The 'presented' callback then looks at the input and commit
timestamps, and compares it with the presented timestamp.
The delay is logged at INFO when the delay was less than one frame
interval, at WARN when it was one frame interval, and at ERR when it
was two or more frame intervals.
We also update statistic counters that we log when foot is shut down.
We may have many windows open, which tries to update/change the
xcursor at various points.
Track which cursor is currently loaded, regardless of which window it
was set by. If someone tries to load that very same xcursor again,
simply skip it.
One example is when we've moused over a window that does *not* have
keyboard focus, and then the user clicks, or by some other mean gives
that window keyboard focus. In many cases it will then try to set the
same cursor again (most of the times, the cursor is the same
regardless of keyboard focus, but not always).
When there hasn't been a timeout (or in our case, there was a timeout,
but we reset the timer before we got to the read()), read() returns,
not 0, but -1 with errno == EAGAIN.
Since we cancel the timers every now and then, there's a (small)
chance that one handler cancels a timer that has triggered in the same
epoll() iteration.
When this happens, read() blocks.
Fix by making the timer FDs non-blocking, and simply returning when we
read 0 bytes.
This will trigger e.d. keyboard_leave() and wl_pointer_leave() events,
which ensures there aren't any references to the destroyed window from
the global wayland struct.
Call wl_display_roundtrip() to trigger those events *before* we
destroy the window.
