Add support for double precision audio #34

So far I've only seen REAPER running iZotope Rx plugins utilize this.

src/wine-host/bridges/vst2.h

@@ -242,7 +242,8 @@ class Vst2Bridge {
      */
     Win32Thread parameters_handler;
     /**
-     * The thread that handles calls to `processReplacing` (and `process`).
+     * The thread that handles calls to `processReplacing` (and `process` as a
+     * fallback) and `processDoubleReplacing`.
      */
     Win32Thread process_replacing_handler;