Compute moment of inertia for concave and boundary meshes. Resolve #338.

- Mesh inertias can now be computed exactly for well-formed (no holes) non-convex meshes.
- To activate this feature, set `<compiler exactmeshinertia="true">` (defaults to `false`). This default may change in the future.
- Added `<geom shellinertia="true/false">` (defaults to `false`). When true, geom inertia is computed assuming all the mass is concentrated on the surface, and `density` is interpreted as surface density (mass/area). Currently only mesh geoms are supported.

PiperOrigin-RevId: 464368395
Change-Id: I17afd99b9b221c9d24ae951f6e63d5c61ff89820

test/xml/xml_native_reader_test.cc

@@ -294,6 +294,25 @@ TEST_F(UserDataTest, InvalidInertialOrientation) {
   EXPECT_THAT(error.data(), HasSubstr("multiple orientation specifiers for the same field"));
 }
 
+TEST_F(UserDataTest, ReadShellParameter) {
+  static constexpr char xml[] = R"(
+  <mujoco>
+    <asset>
+      <mesh name="example_mesh"
+        vertex="0 0 0  1 0 0  0 1 0  0 0 1"
+        face="0 2 1  2 0 3" />
+    </asset>
+    <worldbody>
+      <geom type="mesh" mesh="example_mesh" shellinertia="true"/>
+    </worldbody>
+  </mujoco>
+  )";
+  std::array<char, 1024> error;
+  mjModel* model = LoadModelFromString(xml, error.data(), error.size());
+  ASSERT_THAT(model, NotNull());
+  mj_deleteModel(model);
+}
+
 TEST_F(UserDataTest, ReadsDamper) {
   static constexpr char xml[] = R"(
   <mujoco>