timespec: Add timespec_to_msec helper

Paralleling timespec_to_nsec, converts to milliseconds.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
[Pekka: added doc about flooring]
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
diff --git a/libweston/compositor.c b/libweston/compositor.c
index 9ded23f..2a9f476 100644
--- a/libweston/compositor.c
+++ b/libweston/compositor.c
@@ -2388,7 +2388,7 @@
 						  output->msc,
 						  presented_flags);
 
-	output->frame_time = stamp->tv_sec * 1000 + stamp->tv_nsec / 1000000;
+	output->frame_time = timespec_to_msec(stamp);
 
 	weston_compositor_read_presentation_clock(compositor, &now);
 	timespec_sub(&gone, &now, stamp);