demux_mkv: fix realvideo timestamp handling

This was broken by the recent commits. Apparently realvideo timestamps
are severely mangled, and Matroska _of course_ doesn't have the sane,
umangled timestamps, but something unusable. The existing unmangling
code in demux_mkv.c didn't output proper timestamps either. Instead,
it was something weird that triggered sorting. Without sorting (it was
disabled by default recently), you'd get decreasing PTS warnings

In order to fix this, steal some code from libavcodec. Basically copy
the contents of rv34_parser.c (with some changes), which makes
everything magically work. (Maybe it would be better to use the
libavcodec parser API, but I don't want to do that just for this. An
alternative idea would be refusing to read files that have realvideo
tracks, and delegate this to demux_lavf.c, but maybe that's too redical
too.)

I wish I hadn't notice this...
video: unbreak --no-correct-pts with demuxers that use DTS
video/filter: fix PIC compile on x86

When using PIC on x86 (eg with hardened toolchains) the ebx register is
reserverd and cannot be used in assembly code.

For vf_eq we allow the compiler to use memory as input.

For vf_noise we temoporarily borrow the ebp register.

This fixes #361.

Signed-off-by: Natanael Copa <ncopa@alpinelinux.org>