Use correct start index for Art-Net in proxy mode

[askask]

Art-Net data doesn't contain a start byte. This is currently only taken into account when using regular LED output.
With this commit it also works when using DMX proxy mode.

[softhack007]

@troyhacks, @netmindz: If I remember correctly, you are regularly working with Art-Net? Would like to get your thoughts on this PR.

[troyhacks]

I think I understand it - it's just aligning incoming Art-Net/E1.31 data so channels from an Art-Net packet hits DMX output on the correct channel.

I haven't used this feature, but I think it's safe to merge. Good catch, @askask!

[github-actions]

Hey! This pull request has been open for quite some time without any new comments now. It will be closed automatically in a week if no further activity occurs.
Thank you for contributing to WLED! ❤️

[netmindz]

I'm not sure WLED is really appropriate for trying to be a an ArtNet/sACN to DMX (wired) convertor, but if has that feature it should certainly map the right values to the right channel.

If we are looking for confirmation of fix before merge with actual hardware I can probably rustle something up.

For future reference I would always recommend using a dedicated esp32 if you want such bridge from wireless to wired DMX, lighting desk send a universe at up to 44 Hz which WLED can't provide so you will lose frames and moving heads might behave erratically when getting a slow and inconsistent update