cocoa_common: track mouse move events when not focused

This is really only important with the OSC and makes the tracking code behave
more consistently with the X11 one.
cocoa_common: enable click-through on the video view

Generate a mouse down event on the first click so that one can interact
with the OSC directly as opposed to wasting the first click in order to focus
the window.
input: lock for accessing struct input_ctx

The previous code was locking only the input queue. That was too weak since it
didn't protect the input_ctx data structure. So remove the locking on the queue
and lock all the public functions that interact with the input_ctx.

The private functions and public functions that do not act on the input_ctx
(there are quite some functions doing mp_cmd manipulations for instance) are
not locked.

Some changes by wm4. Use a recursive mutex, and restructure some code to be
less annoying with locks, such as converting code to single return, or
making use of the recursive mutex.
travis: don't generate docs for ffmpeg/libav

Fixes some breakage with ffmpeg-git
input: comment all default bindings in input.conf

I suspect most users will just copy etc/input.conf when they want to
remap some default bindings. But usually this means the user even copies
bindings he doesn't care about, and it's better if the user maps only
the bindings in his input.conf the user intends to remap.

Comment all bindings in etc/input.conf. Since this file also defines the
builtin defaults and is baked into the mpv binary, we have to do
something to get them anyway, even though they are commented. Do this by
having input.c "uncomment" the bindings in the baked in input.conf. (Of
course this is done only for the builtin config, not configs loaded from
disk.)
audio: fix playback of Musepack SV8 files

This is basically a libavcodec API oddity: it can happen that
avcodec_decode_audio4() returns 0 (meaning 0 bytes were consumed). It
requires you to feed the complete packet again to decode the full
packet, and to successfully decode the following packets.

We ignored this case with the argument that there's the danger of an
endless decode loop (because nothing of that packet is apparently
decoded, so it would retry forever), but change it in order to decode
mpc8 files correctly.

Also add some comments to explain the mess.
keycodes: cosmetics: align indentation
input: add some more X11 multimedia/internet keys

These keys can be found on various "multimedia" and "internet" keyboard.
X defines many keycodes, so I'm not adding all, just what I found on my
own keyboard.

Other key codes can be added on request.
input: stay in current input section if mouse button is down

Normally, moving the mouse outside of the mouse area of an input section
will send mouse events somewhere else (because input section mouse areas
are similar to windows/widgets in real GUI toolkits). This was done even
if a mouse button was held down. This is quite different from how GUI
toolkits behave.

Change the code so that if a mouse button is down, the mouse area of the
current input section can't be left. Releasing the mouse button (while
the mouse pointer is outside of the mouse area) will actually leave the
mouse area.

As a side-effect, this commit also tests more often whether the current
mouse input section is valid. This is needed to make releasing a mouse
button trigger the mouse input section change.
input: print more debug infos, raise loglevel
input: deal with spurious X11 LeaveNotify events

If the mpv window is unfocus, clicking on the OSC should focus the
window (done by the window manager) and allow interaction with the OSC.
But somehow X sends a spurious LeaveNotify event, immediately followed
by an EnterNotify event. This happens at least with IceWM. The result is
that the OSC will disappear (due to receiving MOUSE_LEAVE). The OSC will
stay invisible, because EnterNotify isn't handled, and there's nothing
that could make the OSC appear again.

Solve this by handling EnterNotify. We cause a redundant MOUSE_MOVE
event to be sent, which triggers the code to make the OSC visible. We
have to remove the code from input.c, which ignores redundant mouse move
events.

Since the code ignoring redundant mouse move events is still needed on
Windows, move that code to w32_common.c. The need for this is documented
in the code, also see commit 03fd2fe. (The original idea was to save
some code by having this code in the core, but now it turns out that
this didn't quite work out.)
input: allow window dragging even if mouse is in a mouse area

If the input section is enabled with MP_INPUT_ALLOW_VO_DRAGGING, then
the  VO will be allowed to drag the window with mouse button down +
mouse move even if the mouse is inside the section's mouse area.
osx: use MP_KEY_* instead of MK_* for media keys

In 213ad5d6c I added `MK_*` key bindings overlooking the fact that mpv already
has `MP_KEY_*` for media keys.
input: unlock playloop thread during select

This allows other threads to use mp_input_put_key without blocking if the
playloop is doing the 500ms select call (i.e.: during pause).
Makes Cocoa GUI responsive again (regression since 2d363c3).
mplayer: fix race condition on uninit with Cocoa

NULL out Cocoa's reference to mplayer's input context before deallocating it
x11: add window icon

The png file added to etc/ are taken from the link mentioned in commit
303096b, except that they have been converted to 16 bit, sRGB (with
color profile info dropped, if there was one), and transparent pixels
reset for better compression.

The file x11_icon.bin is generated by gen-x11-icon.sh. I'm adding it to
the git repo directly, because the script requires ImageMagick, and we
don't want to make building even more complicated.

The way how this is done is basically a compromise between effort
required in x11_common.c and in gen-x11-icon.sh. Ideally, x11_icon.bin
would be directly in the format as required by _NET_WM_ICON, but trying
to write the binary width/height values from shell would probably be a
nightmare, so here we go.

The zlib code in x11_common.c is lifted from demux_mkv.c, with some
modifications (like accepting a gzip header, because I don't know how to
make gzip write raw compressed data).
cocoa: enqueue events only if input context is present
cocoa_common: avoid the opengl view to leak it's state

Just because everything is in a single file it doesn't excuse us to have high
coupling between C and ObjC code.
cocoa_common: remove unlocking from fullscreen function

This was added in the past to prevent a deadlock, but is not needed
anymore.
macosx_events: remove duplication

refactoring: extract method `handleMPKey:withMask:`