Make sure sampleTime used in sound updates is multiple of 8

Originally sound updates only happened about every 100ms and
`sampleTime` (or `newSoundTime`) was a multiple of 4096
(`MIXBUFFER_SAMPLES`).
After I changed this to updates every 16ms and made the calculation of
`sampleTime` a lot simpler, it could be any value (as it's current
amount of milliseconds multiplied by 44.1).
It generally seemed to work, but it seems advisable to make it a
multiple of 8 (see also "Fix endless loop when decoding OGGs" commit).
So I round it to the nearest multiple of 8 now. Furthermore I increased
the accuracy when the game has been running for a long time by using
double instead of float, and tried to make sure that `sampleTime` is
always positive (or at least as long as `inTime` is positive).

neo/sound/snd_system.cpp

@@ -793,7 +793,19 @@ int idSoundSystemLocal::AsyncUpdateWrite( int inTime ) {
 		return 0;
 	}
 
-	int sampleTime = inTime * 44.1f;
+	// inTime is in milliseconds and if running for long enough that overflows,
+	// when multiplying with 44.1 it overflows even sooner, so use int64 at first
+	// (and double because float doesn't have good precision at bigger numbers)
+	// and then manually truncate to regular int afterwards - this should at least
+	// prevent sampleTime becoming negative (as long as inTime is not)
+	long long int sampleTime64 = double( inTime ) * 44.1;
+
+	// furthermore, sampleTime should be divisible by 8
+	// (at least by 4 for handling 11kHz samples) so round to nearest multiple of 8
+	sampleTime64 = (sampleTime64 + 4) & ~(long long int)7;
+
+	const int sampleTime = sampleTime64 & INT_MAX;
+
 	int numSpeakers = s_numberOfSpeakers.GetInteger();
 
 	// enable audio hardware caching