Add flushing PIO data

[Problem]
When providing data fast enough, multiple frames could be concatenated
since we only waited 60 microseconds after the last bytes were pushed
to the PIO, not since the last bytes were actually pushed to the
neopixels.

[Solution]
"Flush" the PIO after pushing the last bytes, by waiting until it's
empty.

src/frame.rs

@@ -42,7 +42,8 @@ where
     for (r, g, b) in cons.pop_iter().skip(2).take(len).tuples() {
         ws2812.write(r, g, b);
     }
-
+    // wait for the state machine to write all bytes
+    ws2812.flush();
     // let the neopixels latch on
     Timer::after_micros(60).await;
 }

src/ws2812.rs

@@ -96,6 +96,15 @@ impl<'d, P: Instance, const S: usize> Ws2812<'d, P, S> {
         self.sm.tx().dma_push(self.dma.reborrow(), data).await;
     }
 
+    // see https://github.com/rp-rs/ws2812-pio-rs/blob/944494ca9dad73933f700408c3054c8f14c78998/src/lib.rs#L262-L263
+    pub fn flush(&mut self) {
+        // clear stalled flag first
+        self.sm.tx().stalled();
+        while !self.sm.tx().empty() && !self.sm.tx().stalled() {
+            cortex_m::asm::nop();
+        }
+    }
+
     pub fn write(&mut self, r: u8, g: u8, b: u8) {
         let word = convert_colors_to_word(r, g, b);
         // we need to watch out not to overflow the FIFO, see: