Make `--doctool` imply `--headless` to speed up class reference generation

This also prevents spawning an unnecessary splash screen window
while the class reference is generated.

main/main.cpp

@@ -974,6 +974,11 @@ Error Main::setup(const char *execpath, int argc, char *argv[], bool p_second_ph
 		} else if (I->get() == "--doctool") {
 			// Actually handling is done in start().
 			cmdline_tool = true;
+
+			// `--doctool` implies `--headless` to avoid spawning an unnecessary window
+			// and speed up class reference generation.
+			audio_driver = "Dummy";
+			display_driver = "headless";
 			main_args.push_back(I->get());
 #endif
 		} else if (I->get() == "--path") { // set path of project to start or edit