ws2812: Fix first write making the first pixel green

The first epic_ws2812_write() call will set the first pixel to #008000. This is caused by the GPIO line being pulled down on mode-setting (epic_gpio_set_pin_mode). Wait before writing the values to reset the bus and thus properly set the pixels to the correct colors on first write.

The first LED will still flash green for a very short time. I don't know if there is a workaround for this ...

THIS IS UNTESTED!!! I do not have a neopixel with me right now, so I'd need someone else to confirm whether this works ...

If it works, this MR fixes #181 (closed).

Cc: @lilafisch

epicardium/modules/ws2812.c

@@ -63,14 +63,22 @@ void epic_ws2812_write(uint8_t pin, uint8_t *pixels, uint32_t n_bytes)
 
 	taskENTER_CRITICAL();
 
-	epic_gpio_set_pin_mode(pin, EPIC_GPIO_MODE_OUT);
+	/*
+	 * If the pin was not previously configured as an output, the
+	 * `epic_gpio_set_pin_mode()` call will pull it down which the first
+	 * neopixel interprets as the color `0x008000`.  To fix this, wait a bit
+	 * after mode-setting and then write the new values.
+	 */
+	if ((epic_gpio_get_pin_mode(pin) & EPIC_GPIO_MODE_OUT) == 0) {
+		epic_gpio_set_pin_mode(pin, EPIC_GPIO_MODE_OUT);
+	}
+
+	GPIO_OutClr(pin_cfg);
+	epic_ws2812_delay_ticks(EPIC_WS2812_RESET_TCKS);
 
 	do {
 		epic_ws2812_transmit_byte(pin_cfg, *pixels);
 	} while (++pixels != pixels_end);
 
-	GPIO_OutClr(pin_cfg);
-	epic_ws2812_delay_ticks(EPIC_WS2812_RESET_TCKS);
-
 	taskEXIT_CRITICAL();
 }