video: remove BITMAPINFOHEADER from internal demuxer API

MPlayer traditionally did this because it made sense: the most important
formats (avi, asf/wmv) used Microsoft formats, and many important
decoders (win32 binary codecs) also did. But the world has changed, and
I've always wanted to get rid of this thing from the codebase.

demux_mkv.c internally still uses it, because, guess what, Matroska has
a VfW muxing mode, which uses these data structures natively.
audio: remove WAVEFORMATEX from internal demuxer API

Same as with the previous commit. A bit more involved due to how the
code is written.
demux_mkv: get rid of MS structs

See previous commits. This finally replaces directly reading the file
data into a struct with reading them manually. In theory this is more
portable (no alignment issues and other things). For the most part,
it's nice seeing this gone.