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/acio/mdxf/mdxf.cpp

@@ -53,17 +53,12 @@ static struct {
 
 static bool STATUS_BUFFER_FREEZE = false;
 
-typedef enum {
-    THREAD_MODE = 0,
-    BACKFILL_MODE = 1
-} MDXFBufferFillMode;
-
- /* Decides which method to use for populating ring buffer entries for "padding". Could possibly be made configurable through spicecfg.
+ /* Decides which method to use for populating ring buffer entries for "padding". Overwritten in spicecfg using P4IO Buffer Algorithm option.
     THREAD_MODE: Spins a thread running at THREAD_REFRESH_RATE_HZ which periodically fills the ring buffer with auxiliary entries. Falls back on BACKFILL_MODE
         if the game's refresh rate is fast enough to forego starting the thread.
     BACKFILL_MODE: On every update cycle, fill the ring buffer with entries for the last known state BACKFILL_INTERVAL_MS apart from each other 
         from the time of the last entry to the current time before adding the entry for the current state. */
-static const MDXFBufferFillMode BUFFER_FILL_MODE = THREAD_MODE;
+acio::MDXFBufferFillMode acio::MDXF_BUFFER_FILL_MODE = acio::THREAD_MODE;
 
 typedef enum {
     ARKMDXP4_POLL = 0,
@@ -270,7 +265,7 @@ static bool __cdecl ac_io_mdxf_update_control_status_buffer_impl(int node, MDXFP
             if (!IS_MDXF_ACTIVE) {
                 IS_MDXF_ACTIVE = true;
             }
-            if (BUFFER_FILL_MODE == THREAD_MODE) {
+            if (acio::MDXF_BUFFER_FILL_MODE == acio::THREAD_MODE) {
                 count_calls_from_game();
             }
         }
@@ -463,7 +458,7 @@ void acio::MDXFModule::attach() {
 }
 
 acio::MDXFModule::~MDXFModule() {
-    if (BUFFER_FILL_MODE == THREAD_MODE) {
+    if (acio::MDXF_BUFFER_FILL_MODE == acio::THREAD_MODE) {
         mdxf_thread_stop();
     }
 }