DSP: Add thread priority control

DSP threads, such as the Rubber Band processing, and planned moves of
other processing to buffer threads, such as the Equalizer, FreeSurround,
HRTF, and Downmixing for output, because they all have small output
buffers. Since these buffers drain and fill fast, they should be
processed at a high priority. Hopefully, App Store doesn't complain
about the use of these APIs.

Signed-off-by: Christopher Snowhill <kode54@gmail.com>

Audio/Chain/DSPNode.h

@@ -15,6 +15,8 @@
 
 - (id _Nullable)initWithController:(id _Nonnull)c previous:(id _Nullable)p latency:(double)latency;
 
+- (void)threadEntry:(id _Nullable)arg;
+
 @end
 
 #endif /* DSPNode_h */

Audio/Chain/DSPNode.m

@@ -37,4 +37,14 @@
 	return self;
 }
 
+// DSP threads buffer for low latency, and therefore should have high priority
+- (void)threadEntry:(id _Nullable)arg {
+	@autoreleasepool {
+		NSThread *currentThread = [NSThread currentThread];
+		[currentThread setThreadPriority:0.75];
+		[currentThread setQualityOfService:NSQualityOfServiceUserInitiated];
+		[self process];
+	}
+}
+
 @end