player: add a --dump-stats option

This collects statistics and other things. The option dumps raw data
into a file. A script to visualize this data is included too.

Litter some of the player code with calls that generate these
statistics.

In general, this will be helpful to debug timing dependent issues, such
as A/V sync problems. Normally, one could argue that this is the task of
a real profiler, but then we'd have a hard time to include extra
information like audio/video PTS differences. We could also just
hardcode all statistics collection and processing in the player code,
but then we'd end up with something like mplayer's status line, which
was cluttered and required a centralized approach (i.e. getting the data
to the status line; so it was all in mplayer.c). Some players can
visualize such statistics on OSD, but that sounds even more complicated.
So the approach added with this commit sounds sensible.

The stats-conv.py script is rather primitive at the moment and its
output is semi-ugly. It uses matplotlib, so it could probably be
extended to do a lot, so it's not a dead-end.
vo_opengl: make spline36 default with --vo=opengl-hq
ao_null: add simulated device latency, simulate EOF problems

This EOF problems happen at least with PulseAudio, but since it's hard
to reproduce, let ao_null optionally simulate it.
audio: explicitly document audio EOF condition

This should probably be an AO function, but since the playloop still has
some strange stuff (using the buffered_audio variable instead of calling
ao_get_delay() directly), just leave it and make it more explicit.
audio: add hack against broken pulseaudio EOF condition

This was reported with PulseAudio 2.1. Apparently it still has problems
with reporting the correct delay. Since ao_pulse.c still has our custom
get_delay implementation, there's a possibility that this is our fault,
but this seems unlikely, because it's full of workarounds for issues
like this. It's also possible that this problem doesn't exist on
PulseAudio 5.0 anymore (I didn't explicitly retest it).

The check is general and works for all push based AOs. For pull based
AOs, this can't happen as pull.c implements all the logic correctly.
w32_common: use ToUnicode to translate key input

This replaces translate_key_input with a solution that gives mpv more
control over how keyboard input is converted to unicode. As a result:

- Key up/down events are generated the same way for all keys.
- Dead keys generate their base character instead of being combined with
  the following character.
- Many Ctrl and Ctrl+Alt key combinations that were previously broken
  are fixed, since it's possible to discover the base keys.
- AltGr doesn't produce special characters when mp_input_use_alt_gr is
  false.

This also fixes some logic to do with detecting AltGr and adds proper
UTF-16 decoding.
player: unrangle one aspect of audio EOF handling

For some reason, the buffered_audio variable was used to "cache" the
ao_get_delay() result. But I can't really see any reason why this should
be done, and it just seems to complicate everything.

One reason might be that the value should be checked only if the AO
buffers have been recently filled (as otherwise the delay could go low
and trigger an accidental EOF condition), but this didn't work anyway,
since buffered_audio is set from ao_get_delay() anyway at a later point
if it was unset. And in both cases, the value is used _after_ filling
the audio buffers anyway.

Simplify it. Also, move the audio EOF condition to a separate function.
(Note that ao_eof_reached() probably could/should whether the last
ao_play() call had AOPLAY_FINAL_CHUNK set to avoid accidental EOF on
underflows, but for now let's keep the code equivalent.)
player: add a --loop-file option

Unlike --loop, loops a file instead of the playlist.
player: fix audio EOF check on --no-audio

This considered audio to never reach EOF when audio was disabled,
instead of always being EOF.
manpage: document loop-file property

Was forgotten in commit 1b398e99.
audio: preallocate audio buffers on resize

This avoids too many realloc() calls if the caller is appending to an
audo buffer. This case is actually quite noticeable when using something
that buffers a large amount of audio.
input: handle multi-combinations as key sequences

The input code always supported combinations of multiple keys (even in
MPlayer, although there the code was active really only for mouse
buttons). This was arcance and also made the code more complicated. I
only know of a single person who ever made use of this feature.

Remove this feature, and repurpose some of the support code (e.g.
parsing, display of key combinations, etc.) to handle such multi-
combinations as sequences, instead of keys to be pressed at the same
time. This is much simpler and implements the feature requested in
github issue #718.

This commit will probably cause a bunch of regressions, since the input
handling code has some weird corner cases. I couldn't find any problems
when testing, though.
input: close window when window close button is pressed with --input-test

The window close button is usually mapped to the CLOSE_WIN pseudo-key.
Until now, --input-test treated this pseudo-key like any other key (like
the rest of the input handling code), so you couldn't close the window
in this mode. The manpage had silly instructions and warnings how to
deal with this.

Just always quit when CLOSE_WIN is received, and improve the
instructions.
input: move a function
input: slightly simplify mouse dispatching

I admit I don't really understand anymore why this was needed. Delete it
anyway. It was added with commit 42fa7cbbf.
input: rename a variable

Squashing this with the following commit would be confusing.
input: simplify by not tracking key up/down states

We only need to track key up/down for a single key. There may be some
minor loss of robustness, but this can (probably) happen only if a VO or
user sends strange or complicated sequences of events. In the normal
case, what we do is more than enough. Most simplification comes from the
fact that mpv is not a game console, and users normally execute only one
action at once.
input: keycodes: reorder flags

MP_KEY_EMIT_ON_UP and MP_NO_REPEAT_KEY are not modifiers, just static
flags that some keycodes set.

This is just a cosmetic change.
input: remove minor redundancy
input: don't wakeup core if key repeat is requested but not possible

There's no need to wakeup the core in this situation.