lavu/imgutils: create misc functions for dealing with buffers

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()).