nvk/nak/xfb: handle skipping properly when setting xfb_attr.
nvk/nak/xfb: handle skipping properly when setting xfb_attr.
The code treated 0x00 and 0xff the same, but they aren't,
port over the codegen code.
Fixes GTF-GL45.gtf40.GL3Tests.transform_feedback3.transform_feedback3_skip_components
with zink on nvk