This code is intended for errors in external libraries
when no corresponding error code can be found.
AVERROR_UNKNOWN is too vague for that and looks like AVERROR_BUG.
Move the lavc/imgconvert functions and rename them as follows:
avpicture_get_size -> av_image_get_buffer_size()
avpicture_fill -> av_image_fill_arrays()
avpicture_layout -> av_image_copy_to_buffer()
The new functions have an align parameter, which allows to define the
linesize alignment assumed in the buffer (which is set or read).
The names of the functions are consistent with the lavu/samples API
(av_samples_get_buffer_size(), av_samples_fill_arrays()).
