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/renderer_backend.h

@@ -0,0 +1,47 @@
+// Copyright 2026 DeepMind Technologies Limited
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef MUJOCO_SRC_EXPERIMENTAL_PLATFORM_RENDERER_BACKEND_H_
+#define MUJOCO_SRC_EXPERIMENTAL_PLATFORM_RENDERER_BACKEND_H_
+
+namespace mujoco::platform {
+
+// Describes the configuration of the renderer backend.
+//
+// This is generally a combination of two things. The first describes the
+// high-level rendering engine that sits behind the MuJoCo API (e.g.
+// mjr_render). The second describes the low-level graphics engine that is
+// powering the rendering (e.g. OpenGL, Vulkan, etc.).
+//
+// More details about the backends can be found in renderer.h.
+enum class RendererBackend {
+  // The classic MuJoCo OpenGL renderer.
+  ClassicOpenGl,
+
+  // The Filament-based renderer running on OpenGL.
+  FilamentOpenGl,
+
+  // The Filament-based renderer running on Vulkan.
+  FilamentVulkan,
+
+  // The Filament-based renderer running on WebGL.
+  FilamentWebGl,
+
+  // Similar to FilamentOpenGl, but assumes "headless" rendering.
+  FilamentOpenGlHeadless,
+};
+
+}  // namespace mujoco::platform
+
+#endif  // MUJOCO_SRC_EXPERIMENTAL_PLATFORM_RENDERER_BACKEND_H_