Added a spinlock template as well as a thread work pool class.

Also, optimized shader compilation to happen on threads.

core/thread_work_pool.cpp

@@ -0,0 +1,53 @@
+#include "thread_work_pool.h"
+#include "core/os/os.h"
+
+void ThreadWorkPool::_thread_function(ThreadData *p_thread) {
+
+	while (true) {
+		p_thread->start.wait();
+		if (p_thread->exit.load()) {
+			break;
+		}
+		p_thread->work->work();
+		p_thread->completed.post();
+	}
+}
+
+void ThreadWorkPool::init(int p_thread_count) {
+	ERR_FAIL_COND(threads != nullptr);
+	if (p_thread_count < 0) {
+		p_thread_count = OS::get_singleton()->get_processor_count();
+	}
+
+	thread_count = p_thread_count;
+	threads = memnew_arr(ThreadData, thread_count);
+
+	for (uint32_t i = 0; i < thread_count; i++) {
+		threads[i].exit.store(false);
+		threads[i].thread = memnew(std::thread(ThreadWorkPool::_thread_function, &threads[i]));
+	}
+}
+
+void ThreadWorkPool::finish() {
+
+	if (threads == nullptr) {
+		return;
+	}
+
+	for (uint32_t i = 0; i < thread_count; i++) {
+		threads[i].exit.store(true);
+		threads[i].start.post();
+	}
+	for (uint32_t i = 0; i < thread_count; i++) {
+		threads[i].thread->join();
+		memdelete(threads[i].thread);
+	}
+
+	memdelete_arr(threads);
+	threads = nullptr;
+}
+
+ThreadWorkPool::~ThreadWorkPool() {
+
+	finish();
+}