Change specularAmbientOcclusion from boolean to enum (#2352)

The user can now choose amongst 3 specular AO methods:
- None, specAO is off
- Simple, specAO is inferred from roughness and diffuse AO
- Bent normals, specAO is computed accurately from cone intersections

The last method is more expensive but produces the best results.

This change also fixes a few issues:
- Rename materialRefraction() and materialRefractionType() for
  consistency
- Fixes user time in shaders

samples/sample_full_pbr.cpp

@@ -362,7 +362,7 @@ static void setup(Engine* engine, View* view, Scene* scene) {
             .name("DefaultMaterial")
             .material(shader.c_str())
             .multiBounceAmbientOcclusion(true)
-            .specularAmbientOcclusion(true)
+            .specularAmbientOcclusion(MaterialBuilder::SpecularAmbientOcclusion::BENT_NORMALS)
             .shading(Shading::LIT);
 
     if (hasUV) {