Cycles: Change default material to match Blender and EEVEE

Use Principled BSDF instead of Diffuse BSDF. This is a breaking change but
likely does not affect many scenes significantly. It adds some specularity
when an object does not have a a material assigned.

Fix #142538

Pull Request: https://projects.blender.org/blender/blender/pulls/142703

intern/cycles/scene/shader.cpp

@@ -672,10 +672,8 @@ void ShaderManager::add_default(Scene *scene)
   {
     unique_ptr<ShaderGraph> graph = make_unique<ShaderGraph>();
 
-    DiffuseBsdfNode *diffuse = graph->create_node<DiffuseBsdfNode>();
-    diffuse->set_color(make_float3(0.8f, 0.8f, 0.8f));
-
-    graph->connect(diffuse->output("BSDF"), graph->output()->input("Surface"));
+    PrincipledBsdfNode *bsdf = graph->create_node<PrincipledBsdfNode>();
+    graph->connect(bsdf->output("BSDF"), graph->output()->input("Surface"));
 
     Shader *shader = scene->create_node<Shader>();
     shader->name = "default_surface";

intern/cycles/scene/shader_nodes.cpp

@@ -2550,7 +2550,7 @@ NODE_DEFINE(PrincipledBsdfNode)
   SOCKET_IN_FLOAT(subsurface_ior, "Subsurface IOR", 1.4f);
   SOCKET_IN_FLOAT(subsurface_anisotropy, "Subsurface Anisotropy", 0.0f);
 
-  SOCKET_IN_FLOAT(specular_ior_level, "Specular IOR Level", 0.0f);
+  SOCKET_IN_FLOAT(specular_ior_level, "Specular IOR Level", 0.5f);
   SOCKET_IN_COLOR(specular_tint, "Specular Tint", one_float3());
   SOCKET_IN_FLOAT(anisotropic, "Anisotropic", 0.0f);
   SOCKET_IN_FLOAT(anisotropic_rotation, "Anisotropic Rotation", 0.0f);