lavfi/crop: support more pixel formats

Signed-off-by: Paul B Mahol <onemda@gmail.com>
avformat/http: support relative url redirection

see also http://tools.ietf.org/html/draft-ietf-httpbis-p2-semantics-17#section-9.5

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>