sdvx: option to disable Live2D (#777)

## Link to GitHub Issue or related Pull Request, if one exists
n/a

## Description of change
Adds an option to disable Live2D. Can be disabled everywhere, or just
during a song.

## Testing
Tested EG final and recent Nabla.

src/spice2x/games/sdvx/sdvx_live2d.cpp

@@ -0,0 +1,72 @@
+#include "sdvx_live2d.h"
+
+// only the Live2D-capable SDVX versions are 64-bit, so the whole feature is
+// compiled out of 32-bit builds.
+#ifdef SPICE64
+
+#include <string>
+
+#include "hooks/graphics/graphics.h"
+#include "launcher/logger.h"
+#include "util/logging.h"
+
+namespace games::sdvx {
+
+    // Live2D in-game scene detection (for the -sdvxnolive2d "ingame" option).
+    //
+    // the game logs scene transitions as "I:Attach: in <SCENE>" / "I:Detach: in
+    // <SCENE>". several scenes correspond to in-song gameplay (with the heavy
+    // Live2D rendering); we watch those log lines and keep the shared flag
+    // the d3d9 backend reads up to date. the hook never alters the log output
+    // (always returns false).
+    static bool live2d_scene_log_hook(
+            void *user, const std::string &data, logger::Style style, std::string &out) {
+        // any of these scenes counts as in-song gameplay (different play modes)
+        static const char *const gameplay_scenes[] = {
+            "in ALTERNATIVE_GAME_SCENE",
+            "in MEGAMIX_GAME_SCENE",
+            "in MEGAMIX_BATTLE",
+            "in BATTLE_GAME_SCENE",
+            "in AUTOMATION_GAME_SCENE",
+            "in ARENA_GAME_SCENE",
+        };
+        bool in_gameplay_scene = false;
+        for (const auto *scene : gameplay_scenes) {
+            if (data.find(scene) != std::string::npos) {
+                in_gameplay_scene = true;
+                break;
+            }
+        }
+        if (!in_gameplay_scene) {
+            return false;
+        }
+        // note: log messages here must NOT contain any matched scene token, else
+        // this hook would re-enter itself when the message is pushed.
+        if (data.find("I:Attach: in ") != std::string::npos) {
+            if (!GRAPHICS_SDVX_LIVE2D_IN_GAMEPLAY.exchange(true, std::memory_order_relaxed)) {
+                log_info("sdvx", "Live2D skip: entering gameplay");
+            }
+        } else if (data.find("I:Detach: in ") != std::string::npos) {
+            if (GRAPHICS_SDVX_LIVE2D_IN_GAMEPLAY.exchange(false, std::memory_order_relaxed)) {
+                log_info("sdvx", "Live2D skip: leaving gameplay");
+            }
+        }
+        return false;
+    }
+
+    void live2d_scene_detection_init() {
+        static bool installed = false;
+        if (installed) {
+            return;
+        }
+        installed = true;
+        // the logger's hook list is a persistent static, so registering here is
+        // safe even though this runs before logger::start(). we intentionally do
+        // NOT log a confirmation now: at this point the log file isn't open yet
+        // and the message would be dropped. the entering/leaving-gameplay lines
+        // above provide runtime confirmation once the logger is running.
+        logger::hook_add(live2d_scene_log_hook, nullptr);
+    }
+}
+
+#endif // SPICE64

src/spice2x/games/sdvx/sdvx_live2d.h

@@ -0,0 +1,14 @@
+#pragma once
+
+namespace games::sdvx {
+
+#ifdef SPICE64
+    // installs the Live2D in-game scene-detection log hook used by the
+    // -sdvxnolive2d "ingame" option. does not require the SDVX game module to
+    // be attached, so it can be enabled purely from the launcher option.
+    // only the Live2D-capable SDVX versions are 64-bit, so this is compiled out
+    // of 32-bit builds.
+    void live2d_scene_detection_init();
+#endif
+
+}