Use runtime enum for the renderer backend instead of preprocessor macros.

TESTED:
  studio:vulkan
  studio:vulkan --offscreen_mode=true
  studio:opengl
  studio:opengl_egl
  studio:classic
  studio:classic --offscreen_mode=true
  studio:server
  robot_sim/frontend:debug_server
  pyviewer:example
  pyviewer:example --define=mujoco_gl=vulkan
PiperOrigin-RevId: 864768322
Change-Id: I4478e0115b3807060d60d98b4ddfd63da9389aac

src/experimental/platform/window.h

@@ -20,6 +20,8 @@
 #include <string>
 #include <string_view>
 
+#include "experimental/platform/renderer_backend.h"
+
 #if __has_include("third_party/GL/gl/include/EGL/egl.h")
 #define MUJOCO_STUDIO_EGL_SUPPORTED 1
 #include "third_party/GL/gl/include/EGL/egl.h"
@@ -35,16 +37,8 @@ namespace mujoco::platform {
 // handles events from the window.
 class Window {
  public:
-  // Configures the window for the specified rendering backend.
-  enum RenderConfig {
-    kClassicOpenGL,
-    kFilamentVulkan,
-    kFilamentOpenGL,
-    kFilamentWebGL,
-  };
-
   struct Config {
-    RenderConfig render_config = kClassicOpenGL;
+    RendererBackend renderer_backend = RendererBackend::ClassicOpenGl;
     bool enable_keyboard = true;
     bool load_fonts = true;
     bool offscreen_mode = false;