Enhancement: Align payload to pixel boundaries in sendFrame for custom formats

[travisbreaks]

Nice library! The zero-copy buffer design is solid.

The Idea

In `sendFrame()`, the packetization loop uses `MAX_PAYLOAD_LENGTH` (1440) as the chunk size. This works perfectly for RGB (1440/3 = 480 pixels) and RGBW (1440/4 = 360 pixels).

But for custom `bytesPerPixel` values (say 5 for RGBWW or unusual formats), the payload could split mid-pixel. A receiver that processes pixels sequentially would get a partial pixel at the end of one packet and the remainder at the start of the next.

Suggested Fix

Round down to the nearest pixel boundary:

int maxPayloadForFormat = (DdpProtocol.MAX_PAYLOAD_LENGTH / bytesPerPixel) * bytesPerPixel;
int payloadLength = Math.min(remainingBytes, maxPayloadForFormat);

Not a problem for the standard RGB/RGBW use cases, but makes the generic `sendFrame()` method more robust.