Fix #105803: Cycles slow light tree build when previewing shader nodes

When linking a texture directly to the material output, it's likely being
done for the purpose of previewing. In that case, bias the heuristic towards
not building a light tree, as it's likely not needed and slow on dense meshes.

intern/cycles/scene/shader.cpp

@@ -153,6 +153,16 @@ static float3 output_estimate_emission(ShaderOutput *output, bool &is_constant)
       estimate *= node->get_float(strength_in->socket_type);
     }
 
+    /* Lower importance of emission nodes from automatic value/color to shader
+     * conversion, as these are likely used for previewing and can be slow to
+     * build a light tree for on dense meshes. */
+    if (node->type == EmissionNode::get_node_type()) {
+      EmissionNode *emission_node = static_cast<EmissionNode *>(node);
+      if (emission_node->from_auto_conversion) {
+        estimate *= 0.1f;
+      }
+    }
+
     return estimate;
   }
   else if (node->type == LightFalloffNode::get_node_type() ||

intern/cycles/scene/shader_graph.cpp

@@ -260,6 +260,7 @@ void ShaderGraph::connect(ShaderOutput *from, ShaderInput *to)
 
     if (to->type() == SocketType::CLOSURE) {
       EmissionNode *emission = create_node<EmissionNode>();
+      emission->from_auto_conversion = true;
       emission->set_color(one_float3());
       emission->set_strength(1.0f);
       convert = add(emission);

intern/cycles/scene/shader_nodes.h

@@ -723,6 +723,8 @@ class EmissionNode : public ShaderNode {
   NODE_SOCKET_API(float3, color)
   NODE_SOCKET_API(float, strength)
   NODE_SOCKET_API(float, surface_mix_weight)
+
+  bool from_auto_conversion = false;
 };
 
 class BackgroundNode : public ShaderNode {