Cater for the case where another state change occurs while the ISR is running.

Thank you peterhinch

examples/rp2/pio_quadrature_encoder.py

@@ -66,6 +66,7 @@ lut_index = 0
 
 def encoder_state_changed_irq_handler(sm):
     global counter, direction, lut_index
+    while sm.rx_fifo():
         lut_index |= sm.get() & 3
         counter += state_look_up_table[lut_index]
         if state_look_up_table[lut_index] != 0: