demux_mkv: enable parsing for VP9

VP9 packets can contain 2 frames in some video packets (from which 1
frame is invisible). Due to a design mismatch between libvpx and the
libavcodec vp9 decoder, libvpx can take the "full" packets, but lavc vp9
can not. The consequence is that we have to split the packets if we want
to feed them to the lavc codec.

This is not entirely correct yet: timestamp handling is missing.
--demuxer=lavf and ffmpeg native utilities have the same problem. We can
fix this only once the ffmpeg VP9 parser is fixed.
terminal: always use SA_RESTART with sigaction()

One problem is that for example stdio functions won't restart syscalls
manually, and instead treat EINTR as an error. So passing SA_RESTART is
the only sane thing to do, unless you have special requirements, which
we don't.
build: conditionally use -Wempty-body

-Wempty-body is not available on all gcc versions but we were using it
unconditionally. Also remove the usage from the clang case. clang still
defines `__GNUC__` so it still gets all the gcc specific flags.

This should fix the build on systems with older gcc versions like OpenBSD which
still comes bundled with gcc 4.2 for license issues.
build: use -pthread when available

Our code currently tries to link -lpthread and adds stuff like -D_REENTRANT
based on the target platform.

GCC actually supports to just pass a -pthread compiler and linker flag that
will automatically enable threading and define the correct symbols for the
platform, so let's try to just use that as our first choice.

clang also supports -pthread but it must be used only as a compiler flag,
so we also take care of that scenario with this commit.
TOOLS: move autocrop.lua to TOOLS/lua
in anticipation of more lua scripts
TOOLS: add lua/drc-control
allows live insertion/deletion/adjustment of drc audio filter
TOOLS: indent lua scripts by 4 spaces instead of 3
vf_vapoursynth: keep pixel aspect ratio

Make the filter apply the pixel aspect ratio of the input to the output.
This is more useful than forcing 1:1 PAR when playing anamorphic video
such as DVDs.

VapourSynth itself actually allows passing through the aspect ratio, but
it's in a not very useful form for us: it's per video-frame instead of
constant (i.e in VSVideoInfo). As long as we don't have a way to allow a
filter to spontaneously change output parameters, we can't use this.
(And I don't really feel like making this possible.)
vf_vapoursynth: use official AbsoluteTime property

I thought the "_" in "_AbsoluteTime" was part of the documentation
markup.

This still doesn't help us with VS filters that change timing;
apparently you must use frame durations instead.
client API: add chapter change event

Also works for mpv_observe_property() on the "chapter" property.
cocoa: fix native fullscreen

This fixes a couple of issues with the Cocoa `--native-fs` mode, primarily:

 - A ghost titlebar at the top of the screen in full screen:

   This was caused by the window constraining code kicking in during
   fullscreen. Simply returning the unconstrained rect from the constraining
   method fixes the problem.

 - Incorrect behavior when using the titlebar buttons to enter/exit
   fullscreen, as opposed to the OSD button.

   This was caused by mpv's internal fullscreen state going out of sync with
   the NSWindow's one. This was the case because `toggleFullScreen:`
   completely bypassed the normal event flow that mpv expects.

Signed-off-by: Ryan Goulden <percontation@gmail.com>

Change style for mpv, simplify and refactor some of the constraining code.

Signed-off-by: Stefano Pigozzi <stefano.pigozzi@gmail.com>
cocoa: cosmetics: use NSMakeSize
cocoa: move sync section assignments in vo_control
cocoa: add constraints to prevent the window to exit the screen

Previously the window could be made to completly exit the screen with a
combination or moving it close to an edge and halving it's size (via cmd+0).

This commit address the problem in the most simple way possibile by
constraining the window to the closest edge in these edge cases.
cocoa: disallow the titlebar to exit the visibleFrame on the bottom edge
cocoa: use mainScreen as a fallback for window constraining
cocoa: refactor sync scheduling to a function

This extracts the scheduling logic to a single function which is nicer to keep
it consistent.

Additionally make sure we don't schedule sync operations from a sync operation
itself since that could cause deadlocks (even if it should not be happening
with the current code).
TOOLS: make youtube-dl_mpv.sh executable
video: don't drop last frame when deinterlacing with yadif

Or in other words, add support for properly draining remaining frames
from video filters. vf_yadif is buffering at least one frame, and the
buffered frame was not retrieved on EOF.

For most filters, ignore this for now, and just adjust them to the
changed semantics of filter_ext. But for vf_lavfi (used by vf_yadif),
real support is implemented. libavfilter handles this simply by passing
a NULL frame to av_buffersrc_add_frame(), so we just have to make
mp_to_av() handle NULL arguments.

In load_next_vo_frame(), we first try to output a frame buffered in the
VO, then the filter, and then (if EOF is reached and there's still no
new frame) the VO again, with draining enabled. I guess this was
implemented slightly incorrectly before, because the filter chain still
could have had remaining output frames.
vf_lavfi: distinguish real errors from other states

Don't just hide real errors, should a filter return them.