allow passing DMAChannel in ctor

enables pre-alloc/pre-config

WS2812Serial.h

 
 class WS2812Serial {
 public:
-	constexpr WS2812Serial(uint16_t num, void *fb, void *db, uint8_t pin, uint8_t cfg) :
+	constexpr WS2812Serial(uint16_t num, void *fb, void *db, uint8_t pin, uint8_t cfg, DMAChannel* dma_chan=nullptr) :
 		numled(num), pin(pin), config(cfg),
-		frameBuffer((uint8_t *)fb), drawBuffer((uint8_t *)db) {
+		frameBuffer((uint8_t *)fb), drawBuffer((uint8_t *)db), dma(dma_chan) {
 	}
 	bool begin();
 	void setPixel(uint32_t num, uint32_t color) {