audio: asio "downmix" 7.1 to stereo (#736)

## Link to GitHub Issue or related Pull Request, if one exists
#730 

## Description of change
Add an option that extracts channels from 7.1 ASIO and presents to 2-ch
ASIO. Same idea as the SDVX 2ch fix except for gitadora we are
duplicating the center channel to front.

## Testing
Tested SDVX and GFDM Arena.

src/spice2x/launcher/options.cpp

@@ -1086,10 +1086,7 @@ static const std::vector<OptionDefinition> OPTION_DEFINITIONS = {
     {
         .title = "GitaDora Two Channel Audio",
         .name = "2ch",
-        .desc = "Attempt to reduce audio channels down to just two channels.\n\n"
-            "Arena Model: downmixes 7.1 to stereo using the AC-4 stereo downmix coefficients "
-            "(ETSI TS 103 190-1). The WASAPI Stereo Downmix option, if set, overrides this "
-            "algorithm.",
+        .desc = "Attempt to reduce audio channels down to just two channels.",
         .type = OptionType::Bool,
         .game_name = "GitaDora",
         .category = "Game Options",
@@ -1985,6 +1982,26 @@ static const std::vector<OptionDefinition> OPTION_DEFINITIONS = {
             {"side", "Side channels only"},
         },
     },
+    {
+        // AsioDownmixToStereo
+        .title = "ASIO 7.1 to Stereo Downmix",
+        .name = "asiodownmix",
+        .desc = "Extracts a single stereo channel pair from a multi-channel ASIO output, "
+            "mapping the selected pair to the device's first two channels.\n\n"
+            "Channel pairs assume a standard 7.1 ASIO layout (0-indexed):\n\n"
+            "front: channels 0/1 (front left/right).\n\n"
+            "center: channel 2 (front center, duplicated to both outputs).\n\n"
+            "rear: channels 4/5 (rear left/right).\n\n"
+            "side: channels 6/7 (side left/right).",
+        .type = OptionType::Enum,
+        .category = "Audio",
+        .elements = {
+            {"front", "Front channels (0/1)"},
+            {"center", "Center channel (2)"},
+            {"rear", "Rear channels (4/5)"},
+            {"side", "Side channels (6/7)"},
+        },
+    },
     {
         // VolumeBoost
         .title = "WASAPI/ASIO Boost Audio Volume",