lagarith: pad RGB buffer by 1 byte.

For left HFYU prediction, we predict from the buffer buf+1 using 8- or
16-byte reads. This means that aligning the buffer by 16 bytes is in
itself not sufficient, because if the width itself is 16- or 8-byte
aligned, the buffer will not be padded, and thus a read of size 16 at
buf+1 will overflow boundaries at the right edge. Padding the buffer by
1 byte is sufficient to not overflow its boundaries.

Fixes bug 342.
