MDX: Exposed P4IO Buffer Filling Mode to Spicecfg (#463)

## Description of change
Extension of #460 
An option was exposed to `spicecfg` (`P4IOBufferMode`) for toggling
between the Thread and Backfill buffer fill implementations from the
prior PR.

## Testing
- Verified the option properly toggled between the two implementations

src/spice2x/launcher/options.h

@@ -241,6 +241,7 @@ namespace launcher {
             IIDXRecDisable,
             MidiAlgoVer,
             MidiNoteSustain,
+            DDRP4IOBufferMode,
             InputRequiresFocus,
             NostalgiaPoke,
             ForceBackBufferCount,