It seems like the raw data version of the kitty graphics transmit
operation is meant to be unassociated (aka straight) alpha, though I
can't find any definitive documentation either way- but in any case
unassociated alpha is more common in image formats and makes the
handling easier for the rest of it.
Also removed a redundant call to `decode_frame_config`, since it's
called implicitly when we call `decode_frame` right after.
- move wuffs code from src/ to pkg/
- eliminate stray debug logs
- make logs a warning instead of an error
- remove initialization of some structs to zero
