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/hooks/graphics/backends/d3d9/d3d9_live2d.h

@@ -0,0 +1,44 @@
+#pragma once
+
+#include <windows.h>
+#include <d3d9.h>
+
+// SDVX Live2D draw-skip support for the D3D9 backend.
+//
+// SDVX renders its Live2D navigator / in-song character through a fixed set of
+// shaders. when the skip is active (see graphics_sdvx_live2d_should_skip)
+// the matching draw calls are dropped to save GPU. shaders are identified by a
+// stable hash of their D3D9 bytecode (object pointers vary per run, the bytecode
+// does not). the hashes were captured with the draw-call fingerprinting tool.
+//
+// every entry point is a no-op unless the feature is enabled (mode != Off), and
+// d3d9 rendering for a device is single-threaded, so none of this needs locking.
+namespace d3d9_live2d {
+
+#ifdef SPICE64
+
+    // record a shader's bytecode fingerprint at creation time
+    void on_create_vertex_shader(IDirect3DVertexShader9 *shader, const DWORD *func);
+    void on_create_pixel_shader(IDirect3DPixelShader9 *shader, const DWORD *func);
+
+    // remember the currently-bound shaders
+    void on_set_vertex_shader(IDirect3DVertexShader9 *shader);
+    void on_set_pixel_shader(IDirect3DPixelShader9 *shader);
+
+    // true if the current draw call should be dropped (skip active AND the bound
+    // shaders identify it as SDVX Live2D)
+    bool should_skip_draw();
+
+#else // !SPICE64
+
+    // only the Live2D-capable SDVX versions are 64-bit; on 32-bit every entry point
+    // compiles away to nothing, so the d3d9 device hooks need no #ifdefs at their
+    // call sites.
+    inline void on_create_vertex_shader(IDirect3DVertexShader9 *, const DWORD *) {}
+    inline void on_create_pixel_shader(IDirect3DPixelShader9 *, const DWORD *) {}
+    inline void on_set_vertex_shader(IDirect3DVertexShader9 *) {}
+    inline void on_set_pixel_shader(IDirect3DPixelShader9 *) {}
+    inline bool should_skip_draw() { return false; }
+
+#endif // SPICE64
+}