Add VisualServer methods to get the video adapter name and vendor

These methods can be used in scripts to retrieve the OpenGL
`GL_RENDERER` and `GL_VENDOR` strings (respectively).

This closes #28404.

drivers/dummy/rasterizer_dummy.h

@@ -742,6 +742,8 @@ public:
 	int get_captured_render_info(VS::RenderInfo p_info) { return 0; }
 
 	int get_render_info(VS::RenderInfo p_info) { return 0; }
+	String get_video_adapter_name() const { return String(); }
+	String get_video_adapter_vendor() const { return String(); }
 
 	static RasterizerStorage *base_singleton;