imgutils: generalize linesize computation for bitstream formats

Make it a subcase of the general algorithm used for the non-bitstream
case. Simplify, and make av_image_get_linesize() and
av_image_fill_linesizes() correctly return the right value when plane
!= 0.

In particular fix a crash occurring with:
-vf format=monow,showinfo,format=monow.
