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>

plugins/win-wasapi/win-wasapi.cpp

@@ -8,6 +8,7 @@
 #include <util/windows/WinHandle.hpp>
 #include <util/windows/CoTaskMemPtr.hpp>
 #include <util/threading.h>
+#include <util/util_uint64.h>
 
 using namespace std;
 
@@ -464,8 +465,8 @@ bool WASAPISource::ProcessCaptureData()
 		data.timestamp = useDeviceTiming ? ts * 100 : os_gettime_ns();
 
 		if (!useDeviceTiming)
-			data.timestamp -= (uint64_t)frames * 1000000000ULL /
-					  (uint64_t)sampleRate;
+			data.timestamp -= util_mul_div64(frames, 1000000000ULL,
+							 sampleRate);
 
 		obs_source_output_audio(source, &data);