feat(IfcImporter): add doubleSidedMaterials option

Some exporters (e.g. certain Revit pipelines) produce geometry whose
winding is not consistent, so front-face-only rendering can hide those
faces. Add an opt-in doubleSidedMaterials flag on IfcImporter that makes
the generated materials render both sides (renderedFaces TWO). Defaults
to false to keep the previous front-face behavior.

Refs #233

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: agviegas

packages/fragments/src/Importers/IfcImporter/index.ts

@@ -130,6 +130,16 @@ export class IfcImporter {
    */
   replaceSiteElevation = true;
 
+  /**
+   * Whether the generated materials should render both faces (double-sided)
+   * instead of just the front face.
+   * @remarks Some exporters (e.g. certain Revit pipelines) produce geometry
+   * whose winding is not consistent, so front-face-only rendering can hide
+   * those faces. Enable this to render both sides. Defaults to false
+   * (front-face only) to match the previous behavior.
+   */
+  doubleSidedMaterials = false;
+
   /**
    * If set, ignores the items that are further away to the origin than this value.
    * Keep in mind that if your IFC is correctly georreferenced, this value should never

packages/fragments/src/Importers/IfcImporter/src/geometry/index.ts

@@ -466,15 +466,11 @@ export class IfcGeometryProcessor {
 
       materialsLocalIds.push(nextId++);
 
-      TFB.Material.createMaterial(
-        builder,
-        r,
-        g,
-        b,
-        a,
-        TFB.RenderedFaces.ONE,
-        0,
-      );
+      const renderedFaces = this._serializer.doubleSidedMaterials
+        ? TFB.RenderedFaces.TWO
+        : TFB.RenderedFaces.ONE;
+
+      TFB.Material.createMaterial(builder, r, g, b, a, renderedFaces, 0);
     }
 
     const materials = builder.endVector();