Use size_t rather than int64_t as the array index in AudioProvider::GetAudioWithVolume

In 32-bit builds the int64_t index resulted in two (inlined) calls to
allmul per audio sample, which took 25-50% of the total CPU time
involved in playing audio from the RAM cache.

Originally committed to SVN as r6900.

aegisub/src/audio_provider.cpp

@@ -76,7 +76,7 @@ void AudioProvider::GetAudioWithVolume(void *buf, int64_t start, int64_t count,
 		int value;
 
 		// Modify
-		for (int64_t i=0;i<count;i++) {
+		for (size_t i = 0; i < (size_t)count; ++i) {
 			value = (int)(buffer[i]*volume+0.5);
 			if (value < -0x8000) value = -0x8000;
 			if (value > 0x7FFF) value = 0x7FFF;