bugfix: reclaim thread stack explicitly by using detached threads

Otherwise the VmData size will keep increasing in multi-threads mode.
Although the increasing VmData size doesn't mean there is enormous
memory leak, it does show that the thread stack is not handled properly
when threads exit.

djvu.c

@@ -531,7 +531,10 @@ static int reflowPage(lua_State *L) {
 	kctx->src = src;
 	if (kctx->precache) {
 		pthread_t rf_thread;
-		pthread_create(&rf_thread, NULL, k2pdfopt_reflow_bmp, (void*) kctx);
+		pthread_attr_t attr;
+		pthread_attr_init(&attr);
+		pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED);
+		pthread_create(&rf_thread, &attr, k2pdfopt_reflow_bmp, (void*) kctx);
 	} else {
 		k2pdfopt_reflow_bmp(kctx);
 	}

pdf.c

@@ -615,7 +615,10 @@ static int reflowPage(lua_State *L) {
 	kctx->src = src;
 	if (kctx->precache) {
 		pthread_t rf_thread;
-		pthread_create( &rf_thread, NULL, k2pdfopt_reflow_bmp, (void*) kctx);
+		pthread_attr_t attr;
+		pthread_attr_init(&attr);
+		pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED);
+		pthread_create( &rf_thread, &attr, k2pdfopt_reflow_bmp, (void*) kctx);
 	} else {
 		k2pdfopt_reflow_bmp(kctx);
 	}