Modernize Thread

- Based on C++11's `thread` and `thread_local`
- No more need to allocate-deallocate or check for null
- No pointer anymore, just a member variable
- Platform-specific implementations no longer needed (except for the few cases of non-portable functions)
- Simpler for `NO_THREADS`
- Thread ids are now the same across platforms (main is 1; others follow)

modules/cvtt/image_compress_cvtt.cpp

@@ -267,12 +267,13 @@ void image_compress_cvtt(Image *p_image, float p_lossy_quality, Image::UsedChann
 		job_queue.num_tasks = static_cast<uint32_t>(tasks.size());
 
 		for (int i = 0; i < num_job_threads; i++) {
-			threads_wb[i] = Thread::create(_digest_job_queue, &job_queue);
+			threads_wb[i] = memnew(Thread);
+			threads_wb[i]->start(_digest_job_queue, &job_queue);
 		}
 		_digest_job_queue(&job_queue);
 
 		for (int i = 0; i < num_job_threads; i++) {
-			Thread::wait_to_finish(threads_wb[i]);
+			threads_wb[i]->wait_to_finish();
 			memdelete(threads_wb[i]);
 		}
 	}