Cycles: speed up light tree build with many instanced mesh lights

by building a subtree for each unique mesh light.
The top-level light tree contains two types of emitters: lamps (local or distant) and mesh lights (emissive objects). Each unique mesh light, regardless of whether it's instanced or not, has a subtree. The leaf node of such subtrees only contain triangles. During traversal, we first traverse the top-level tree until we pick an emitter. If this emitter is a mesh light, we transform it to its local space and continue traversing its subtree until we pick a triangle.

@@ -64,3 +64,3 @@
 KERNEL_DATA_ARRAY(KernelLightTreeNode, light_tree_nodes)
 KERNEL_DATA_ARRAY(KernelLightTreeEmitter, light_tree_emitters)
-KERNEL_DATA_ARRAY(uint, light_to_tree)
+KERNEL_DATA_ARRAY(uint, lamp_to_tree)

@@ -1370,6 +1370,13 @@ using BoundingCone = struct BoundingCone {
   float theta_e;
 };
 
+using LightTreeNodeType = enum LightTreeNodeType {

@@ -1404,3 +1413,2 @@
 
-  /* The location in the lights or triangles array. */
-  int prim_id;
+  union {

@@ -156,3 +190,1 @@
-                            it is an inner node. */
-  int first_emitter_index;               /* Leaf nodes contain an index to first emitter. */
-  unique_ptr<LightTreeNode> children[2]; /* Inner node has two children. */
+  int type = LIGHT_TREE_INNER;

@@ -64,3 +64,3 @@
 KERNEL_DATA_ARRAY(KernelLightTreeNode, light_tree_nodes)
 KERNEL_DATA_ARRAY(KernelLightTreeEmitter, light_tree_emitters)
-KERNEL_DATA_ARRAY(uint, light_to_tree)
+KERNEL_DATA_ARRAY(uint, lamp_to_tree)

@@ -1385,3 +1390,1 @@
-   * Otherwise, it's an index to the node's second child. */
-  int child_index;
-  int num_emitters; /* leaf nodes need to know the number of emitters stored. */
+  LightTreeNodeType type;

@@ -1388,0 +1395,4 @@
+  union {
+    int first_emitter; /* Leaf node stores the index of the first emitter. */
+    int right_child;   /* Inner node stores the index of the right child. */
+    int reference;     /* Instance node refers to the node with the subtree. */

@@ -1407,0 +1422,4 @@
+    struct {
+      int object_id;
+      int node_id;
+    }; /* Mesh Light. */

@@ -64,3 +64,3 @@
 KERNEL_DATA_ARRAY(KernelLightTreeNode, light_tree_nodes)
 KERNEL_DATA_ARRAY(KernelLightTreeEmitter, light_tree_emitters)
-KERNEL_DATA_ARRAY(uint, light_to_tree)
+KERNEL_DATA_ARRAY(uint, lamp_to_tree)

@@ -65,2 +65,3 @@
 KERNEL_DATA_ARRAY(KernelLightTreeEmitter, light_tree_emitters)
-KERNEL_DATA_ARRAY(uint, light_to_tree)
+KERNEL_DATA_ARRAY(uint, lamp_to_tree)
+KERNEL_DATA_ARRAY(uint, mesh_to_tree)