Add an editor setting to configure number of threads for lightmap baking

This can be used to free some CPU cores when baking lightmaps.
When using fewer CPU cores, lightmap baking is slower but background
tasks aren't slowed down as much.

core/os/threaded_array_processor.h

@@ -64,8 +64,10 @@ void process_array_thread(void *ud) {
 	}
 }
 
+// p_num_threads is the number of logical CPU cores to use (0 = use all logical CPU cores available).
+// Negative values subtract from the total number of logical CPU cores available.
 template <class C, class M, class U>
-void thread_process_array(uint32_t p_elements, C *p_instance, M p_method, U p_userdata) {
+void thread_process_array(uint32_t p_elements, C *p_instance, M p_method, U p_userdata, int p_num_threads = 0) {
 	ThreadArrayProcessData<C, U> data;
 	data.method = p_method;
 	data.instance = p_instance;
@@ -74,7 +76,13 @@ void thread_process_array(uint32_t p_elements, C *p_instance, M p_method, U p_us
 	data.elements = p_elements;
 	data.process(0); //process first, let threads increment for next
 
-	int thread_count = OS::get_singleton()->get_processor_count();
+	int thread_count;
+	if (p_num_threads <= 0) {
+		thread_count = MAX(1, OS::get_singleton()->get_processor_count() + p_num_threads);
+	} else {
+		thread_count = p_num_threads;
+	}
+
 	Thread *threads = memnew_arr(Thread, thread_count);
 
 	for (int i = 0; i < thread_count; i++) {
@@ -89,8 +97,9 @@ void thread_process_array(uint32_t p_elements, C *p_instance, M p_method, U p_us
 
 #else
 
+// p_num_threads is intentionally unused when threads are disabled.
 template <class C, class M, class U>
-void thread_process_array(uint32_t p_elements, C *p_instance, M p_method, U p_userdata) {
+void thread_process_array(uint32_t p_elements, C *p_instance, M p_method, U p_userdata, int p_num_threads = 0) {
 	ThreadArrayProcessData<C, U> data;
 	data.method = p_method;
 	data.instance = p_instance;

editor/editor_settings.cpp

@@ -545,6 +545,9 @@ void EditorSettings::_load_defaults(Ref<ConfigFile> p_extra_config) {
 	_initial_set("editors/3d/default_z_near", 0.05);
 	_initial_set("editors/3d/default_z_far", 500.0);
 
+	_initial_set("editors/3d/lightmap_baking_number_of_cpu_threads", 0);
+	hints["editors/3d/lightmap_baking_number_of_cpu_threads"] = PropertyInfo(Variant::INT, "editors/3d/lightmap_baking_number_of_cpu_threads", PROPERTY_HINT_RANGE, "-2,128,1", PROPERTY_USAGE_DEFAULT);
+
 	// 3D: Navigation
 	_initial_set("editors/3d/navigation/navigation_scheme", 0);
 	_initial_set("editors/3d/navigation/invert_y_axis", false);

modules/lightmapper_cpu/lightmapper_cpu.cpp

@@ -36,6 +36,10 @@
 #include "core/project_settings.h"
 #include "modules/raycast/lightmap_raycaster.h"
 
+#ifdef TOOLS_ENABLED
+#include "editor/editor_settings.h"
+#endif
+
 Error LightmapperCPU::_layout_atlas(int p_max_size, Vector2i *r_atlas_size, int *r_atlas_slices) {
 	Vector2i atlas_size;
 	for (unsigned int i = 0; i < mesh_instances.size(); i++) {
@@ -205,7 +209,12 @@ Error LightmapperCPU::_layout_atlas(int p_max_size, Vector2i *r_atlas_size, int
 
 void LightmapperCPU::_thread_func_callback(void *p_thread_data) {
 	ThreadData *thread_data = reinterpret_cast<ThreadData *>(p_thread_data);
-	thread_process_array(thread_data->count, thread_data->instance, &LightmapperCPU::_thread_func_wrapper, thread_data);
+#ifdef TOOLS_ENABLED
+	const int num_threads = EDITOR_GET("editors/3d/lightmap_baking_number_of_cpu_threads");
+#else
+	const int num_threads = 0;
+#endif
+	thread_process_array(thread_data->count, thread_data->instance, &LightmapperCPU::_thread_func_wrapper, thread_data, num_threads);
 }
 
 void LightmapperCPU::_thread_func_wrapper(uint32_t p_idx, ThreadData *p_thread_data) {