lavc: Clarify the behaviour of dimension and format context fields

The AVCodecContext width, height, coded_width, coded_height and format
are used mainly as decoding hints and they get internally overwritten
during the data parsing stage.

Do not assume they match the last AVFrame provided by
avcodec_decode_video2 and assimilated functions since multi-threading
and other frame reordering might make those values to refer to frames
that will be outputted in the future.

CC: libav-stable@libav.org