libobs: Implement and use better scaling function for 64-bit integers

As os_gettime_ns() gets large the current scaling methods, mostly by casting
to uint64_t, may lead to numerical overflows. Sweep the code and use
util_mul_div64() where applicable.

Signed-off-by: Hans Petter Selasky <hps@selasky.org>

test/test-input/sync-pair-aud.c

@@ -2,6 +2,7 @@
 #include <util/bmem.h>
 #include <util/threading.h>
 #include <util/platform.h>
+#include <util/util_uint64.h>
 #include <obs.h>
 
 struct sync_pair_aud {
@@ -54,7 +55,8 @@ static void *sync_pair_aud_thread(void *pdata)
 
 		for (uint64_t i = 0; i < frames; i++) {
 			uint64_t ts =
-				last_time + i * 1000000000ULL / sample_rate;
+				last_time +
+				util_mul_div64(i, 1000000000ULL, sample_rate);
 
 			if (whitelist_time(ts, interval, fps_num, fps_den)) {
 				cos_val += rate * M_PI_X2;