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.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
+
+}