Add very quick and dirty display of backend messages to UI

This is just for testing, has to be properly implemented. With e.g. a
fingerprint reader there is no time to read 'Failed to match
fingerprint' messages and of course the text goes out the circle.
There's also a bunch of issues with timers, e.g. first entering the
wrong fingerprint ~5 times and then typing an incorrect password causes
the UI to go to a weird state.

include/swaylock.h

@@ -13,6 +13,7 @@ enum auth_state {
 	AUTH_STATE_IDLE, // nothing happening
 	AUTH_STATE_VALIDATING, // currently validating password
 	AUTH_STATE_INVALID, // displaying message: password was wrong
+	AUTH_STATE_BACKEND_MSG, // displaying message from backend //TODO: WIP
 };
 
 // Indicator state: status of password buffer / typing letters
@@ -94,6 +95,7 @@ struct swaylock_state {
 	cairo_surface_t *test_surface;
 	cairo_t *test_cairo; // used to estimate font/text sizes
 	enum auth_state auth_state; // state of the authentication attempt
+	char *backend_msg; // message from the backend, will be NULL if empty //TODO: WIP
 	enum input_state input_state; // state of the password buffer and key inputs
 	uint32_t highlight_start; // position of highlight; 2048 = 1 full turn
 	int failed_attempts;

main.c

@@ -1035,8 +1035,12 @@ static void display_in(int fd, short mask, void *data) {
 }
 
 static void handle_backend_text_message(char *msg) {
-	//TODO: show message in UI
 	swaylock_log(LOG_DEBUG, "Received text message from backend: %s", msg);
+
+	//TODO: proper implementation
+	state.auth_state = AUTH_STATE_BACKEND_MSG;
+	state.backend_msg = msg;
+	damage_state(&state);
 }
 
 static void handle_auth_result(bool success) {

render.c

@@ -152,6 +152,8 @@ static bool render_frame(struct swaylock_surface *surface) {
 			text = "Verifying";
 		} else if (state->auth_state == AUTH_STATE_INVALID) {
 			text = "Wrong";
+		} else if (state->auth_state == AUTH_STATE_BACKEND_MSG && state->backend_msg) { //TODO: proper implementation
+			text = state->backend_msg;
 		} else {
 			// Caps Lock has higher priority
 			if (state->xkb.caps_lock && state->args.show_caps_lock_text) {