avio: Use AVERROR_PROTOCOL_NOT_FOUND

When the protocol is missing ffurl_alloc() should return
AVERROR_PROTOCOL_NOT_FOUND instead of AVERROR(ENOENT).

Bug-Id: 577
CC: libav-stable@libav.org
lavc: Edge emulation with dst/src linesize

Allow supporting files for which the image stride is smaller than
the maximum block size + number of subpel mc taps, e.g. a 64x64 VP9
file or a 16x16 VP8 file with -fflags +emu_edge.
lavc: add a VP9 decoder

Originally written by Ronald S. Bultje <rsbultje@gmail.com> with the
help of Clément Bœsch <ubitux@gmail.com>.

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
Signed-off-by: Anton Khirnov <anton@khirnov.net>
wip: Cleanups