Close #5167

Some X servers cannot borrow VLC user privileges to attach to its
shared memory segments. This fallback should fix MIT-SHM with those
X servers. However other users will be able to snoop on the decoded
video frames.

The SDL audio output is gone. The SDL video output is not very useful
and it won't quite work with most SDL video plugins disabled.

SDL is mainly a dependency for SDL_image.

As things stand, we have a format that is neither S/PDIF nor linear,
so change the macros to remove the confusion.

This plugin modifies the output format. It caused a busy loop
and broke libmad on fixed-point architectures (fixes #5151).

The new Peek() function now allocates a block (p_sys->peeked) if the
required size (i_peek) is greater than the size of the playback
segment, and then do a copy of one (or several) segment's data block(s) in it.
In the end, p_sys->peeked might be the concatenation of several segment's data
blocks, but usually it will be NULL.
Signed-off-by: Jean-Paul Saman <jean-paul.saman@m2x.nl>

remove useless prototype
inline vlc identity in the only use case

fix memleak when thread is cancelled

remove FREE_ATT, use do { ... } while(0) for FREE_VALUE
use static const tables for xml handlers
use their size to match handlers with tags
replace {SIMPLE,COMPLEX,UNKNOWN}_CONTENT with a bool

also remove one level of indentation