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/launcher.cpp

@@ -10,6 +10,7 @@
 #include "api/modules/capture.h"
 #include "acio/acio.h"
 #include "acio/icca/icca.h"
+#include "acio/mdxf/mdxf.h"
 #include "api/controller.h"
 #include "avs/automap.h"
 #include "avs/core.h"
@@ -1149,6 +1150,14 @@ int main_implementation(int argc, char *argv[]) {
             rawinput::RAWINPUT_REQUIRE_FOCUS = false;
         }
     }
+    if (options[launcher::Options::DDRP4IOBufferMode].is_active()) {
+        if (options[launcher::Options::DDRP4IOBufferMode].value_text() == "thread") {
+            acio::MDXF_BUFFER_FILL_MODE = acio::THREAD_MODE;
+        } else if (options[launcher::Options::DDRP4IOBufferMode].value_text() == "backfill") {
+            acio::MDXF_BUFFER_FILL_MODE = acio::BACKFILL_MODE;
+        } 
+        // else - default (no value)
+    }
 
     if (options[launcher::Options::MidiAlgoVer].is_active()) {
         if (options[launcher::Options::MidiAlgoVer].value_text() == "legacy") {

src/spice2x/launcher/options.cpp

@@ -2291,6 +2291,24 @@ static const std::vector<OptionDefinition> OPTION_DEFINITIONS = {
         .setting_name = "20",
         .category = "I/O Options",
     },
+    {
+        // DDRP4IOBufferMode
+        .title = "DDR P4IO Buffer Algorithm",
+        .name = "ddrp4iobuffer",
+        .desc =
+            "Remember to restart after changing this value.\n\n"
+            "Sets the algorithm used to populate entries to the buffer of controller polls read by the game.\n\n"
+            "Thread: Starts a thread to periodically insert polls into the buffer (defaults to Backfill if game's refresh rate is at least 120Hz).\n\n"
+            "Backfill: Fills the buffer on each frame with last known state info at short regular intervals up to the current time, then writes the current state.\n\n"
+            "Only has an effect when emulating P4IO (arkmdxp4.dll)",
+        .type = OptionType::Enum,
+        .game_name = "Dance Dance Revolution",
+        .category = "Game Options (Advanced)",
+        .elements = {
+            {"thread", ""},
+            {"backfill", ""},
+        },
+    },
     {
         // InputRequiresFocus
         .title = "Input Requires Focus",

src/spice2x/launcher/options.h

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