spu2-x: stretcher: allow minimum latency of 15ms (was 30ms)

15ms latency is too little most of the time, but if the stars align (light game,
fast system, the correct audio output module - portaudio comes to mind), it
might work well/reasonably, so allow it.

Watch the console for stretcher related messages. If you hear bad audio (clicks
etc) or notice reset/underruns messages, it means the latency is too low. The
optimal behavior (stretching is locked to 1:1) is when a message "stretch: None (1:1)"
shows at the console, which isn't followed by a message "stretch: Dynamic" or
resets or spu2 underruns.

I'm pretty sure such low latency with good performance (mostly locked to 1:1)
was not possible in the past, but it seems possible (sometimes) now. Maybe the
previous "black magic" commit helps

The default is still 100ms which is still fine for most cases.

This commit affects Windows. Linux still has a different minimum, probably
mostly due to UI/constants. Maybe someone should test and change that too.

plugins/spu2-x/src/Windows/Config.cpp

@@ -26,7 +26,7 @@ static const int LATENCY_MAX = 750;
 #endif
 
 static const int LATENCY_MIN = 3;
-static const int LATENCY_MIN_TS = 30;
+static const int LATENCY_MIN_TS = 15;
 
 // MIXING
 int Interpolation = 4;