lavc/rawdec: Use 16-byte line alignment for AV_PIX_FMT_MONOWHITE

The line alignment for 1 bpp raw AV_PIX_FMT_MONOWHITE video (currently
used for AVI) was previously 4 bytes, which generated alignment warning
messages, not only for odd-width files. The alignment is now 16 bytes.

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>