clear screen before poweroff

src/main.c

@@ -77,6 +77,10 @@ int main(void) {
                 backlight_save = r61524_get(0x5a1) & 0x00ff;
                 r61524_set(0x5a1, 0);
 
+                // clear screen
+                dclear(C_BLACK);
+                dupdate();
+
                 shift_state = 0;
                 alpha_state = 0;