utils: also align alpha planes up to full byte size

I'm not entirely sure why this was originally disabled - the comment justifies itself by claiming that the alpha plane does not respect pl_bit_encoding, but the alpha plane is just a normal plane that is sampled like any other plane, via the same pl_sample_src mechanism (which includes the bits scaling).

I think this may have been a legacy all the way from mpv, where bit encoding was handled as part of the 3x3 YUV decoding matrix, but we basically stopped doing that for the majority of inputs a long time ago.

Actually, maybe we should just kill pl_bit_encoding being part of pl_color_repr altogether at this point.