rp2pio: Transfer up to 32 bytes before checking background tasks

@jerryneedell noticed that this problem affected strips short enough
to not use the DMA peripheral, thanks for the hot tip!

Instead of checking for background tasks after every byte transfer,
try up to 32 transfers before attending to background tasks.

This fixes the problem I was seeing on my 5-pixel circuit.

Closes #4135.

Author: jepler

ports/raspberrypi/common-hal/rp2pio/StateMachine.c

@@ -546,15 +546,23 @@ static bool _transfer(rp2pio_statemachine_obj_t *self,
         size_t tx_remaining = out_len;
 
         while (rx_remaining || tx_remaining) {
-            if (tx_remaining && !pio_sm_is_tx_fifo_full(self->pio, self->state_machine)) {
-                *tx_destination = *data_out;
-                data_out++;
-                --tx_remaining;
-            }
-            if (rx_remaining && !pio_sm_is_rx_fifo_empty(self->pio, self->state_machine)) {
-                *data_in = (uint8_t) *rx_source;
-                data_in++;
-                --rx_remaining;
+            for (int i=0; i<32; i++) {
+                bool did_transfer = false;
+                if (tx_remaining && !pio_sm_is_tx_fifo_full(self->pio, self->state_machine)) {
+                    *tx_destination = *data_out;
+                    data_out++;
+                    --tx_remaining;
+                    did_transfer = true;
+                }
+                if (rx_remaining && !pio_sm_is_rx_fifo_empty(self->pio, self->state_machine)) {
+                    *data_in = (uint8_t) *rx_source;
+                    data_in++;
+                    --rx_remaining;
+                    did_transfer = true;
+                }
+                if (!did_transfer) {
+                    break;
+                }
             }
             RUN_BACKGROUND_TASKS;
             if (mp_hal_is_interrupted()) {