replace gettimeofday with clock_gettime

POSIX.1-2008 has marked gettimeofday(2) as obsolete, recommending the
use of clock_gettime(2) instead.

CLOCK_MONOTONIC has been used instead of CLOCK_REALTIME because it is
unaffected by manual changes in the system clock. This makes it better
for our purposes, namely, measuring the difference between two points in
time.

tv_sec has been casted to long in most places since POSIX does not
define the actual type of time_t.

render.h

@@ -4,6 +4,7 @@
 #include "terminal.h"
 #include "fdm.h"
 #include "wayland.h"
+#include "misc.h"
 
 struct renderer;
 struct renderer *render_init(struct fdm *fdm, struct wayland *wayl);