Move filament renderer out of experimental into src/render/filament.

Also:
- renames the main header file to mjrfilament.h.
- moves filament_util under a "support" folder.
PiperOrigin-RevId: 930448415
Change-Id: Iad89756aaf1ddf4c3e67e1afc3a13776f71fa20f

src/render/filament/assets/phong_cube_reflect.mat

@@ -0,0 +1,56 @@
+// Copyright 2025 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.
+
+material {
+    name : phong_cube_reflect,
+    shadingModel : specularGlossiness,
+    flipUV : false,
+    parameters : [
+        { type : float4, name : BaseColorFactor },
+        { type : float, name : SpecularFactor },
+        { type : float, name : GlossinessFactor },
+        { type : float, name : EmissiveFactor },
+        { type : float3, name : UvScale },
+        { type : float, name : Reflectance },
+        { type : samplerCubemap, name : BaseColor },
+        { type : sampler2d, name : Reflection }
+    ],
+    variables : [
+         vertex_pos
+    ]
+}
+
+vertex {
+  void materialVertex(inout MaterialVertexInputs material) {
+    material.vertex_pos = getPosition();
+  }
+}
+
+fragment {
+  void material(inout MaterialInputs material) {
+    vec3 uv = variable_vertex_pos.xyz * materialParams.UvScale;
+    vec2 screen_uv = gl_FragCoord.xy * getResolution().zw;
+
+    prepareMaterial(material);
+    material.baseColor     = materialParams.BaseColorFactor;
+    material.baseColor     *= texture(materialParams_BaseColor, uv);
+    material.specularColor = vec3(materialParams.SpecularFactor);
+    material.glossiness    = materialParams.GlossinessFactor;
+    material.emissive      = vec4(materialParams.EmissiveFactor);
+
+    // Apply reflection.
+    vec4 reflection = texture(materialParams_Reflection, screen_uv);
+    material.baseColor.rgb = mix(material.baseColor.rgb, reflection.rgb, materialParams.Reflectance);
+  }
+}