sd_lavc: factor out bitmap positioning code
input: fix bogus section disabling, add debugging output

The code to remove a section from the active section array wasn't
correct (it should have tried to copy the elements in reverse), so just
replace it with a macro that does the intended thing.

Add some debug output to print the section stack.
demux_lavf: remove unnecessary seek on initialization

This is not needed, and actually completely incorrect.
Add prelimimary (basic, possibly broken) dvdnav support

This readds a more or less completely new dvdnav implementation, though
it's based on the code from before commit 41fbcee. Note that this is
rather basic, and might be broken or not quite usable in many cases.

Most importantly, navigation highlights are not correctly implemented.
This would require changes in the FFmpeg dvdsub decoder (to apply a
different internal CLUT), so supporting it is not really possible right
now. And in fact, I don't think I ever want to support it, because it's
a very small gain for a lot of work. Instead, mpv will display fake
highlights, which are an approximate bounding box around the real
highlights.

Some things like mouse input or switching audio/subtitles stream using
the dvdnav VM are not supported.

Might be quite fragile on transitions: if dvdnav initiates a transition,
and doesn't give us enough mpeg data to initialize video playback, the
player will just quit.

This is added only because some users seem to want it. I don't intend to
make mpv a good DVD player, so the very basic minimum will have to do.
How about you just convert your DVD to proper video files?