Apple: Add pthread implementation of Thread class

This allows Apple platforms to override the default stack size of
a thread in the WorkerThreadPool, which is 512KiB by default.

This must be increased, as SPIRV-Cross, used by the Metal driver, can
use deeply nested stacks, as can debug builds.

drivers/metal/metal_utils.h

@@ -32,6 +32,8 @@
 
 #import <os/log.h>
 
+#import <functional>
+
 #pragma mark - Boolean flags
 
 namespace flags {