vaapi: add vf_vavpp and use it for deinterlacing

Merged from pull request #246 by xylosper. Minor cosmetic changes, some
adjustments (compatibility with older libva versions), and manpage
additions by wm4.

Signed-off-by: wm4 <wm4@nowhere>
vd_lavc: allow process_image to change image format

This will allow GPU read-back with process_image.

We have to restructure how init_vo() works. Instead of initializing the
VO before process_image is called, rename init_vo() to
update_image_params(), and let it update the params only. Then we really
initialize the VO after process_image.

As a consequence of these changes, already decoded hw frames are
correctly unreferenced if creation of the filter chain fails. This
could trigger assertions on VO uninitialization, because it's not
allowed to reference hw frames past VO lifetime.
vaapi: allow GPU read-back with --hwdec=vaapi-copy

This code is actually quite inefficient: it reuses the (slow, simple)
screenshot code. It uses an inefficient method to read the image
(vaGetImage() instead of vaDeriveImage()), allocates new memory for
each frame that is read, and it tries all image formats again each
time.

Also, in my tests it always picked NV12 as image format, which is not
ideal if you actually want to filter the video, and vo_xv can't handle
this format without conversion either.

However, a user confirmed that it worked for him, so everything is fine.
configure: improve pdflatex check
configure: make the pdflatex check use the tempdir
options: make --ass-hinting a choice, instead of using magic numbers

options.c still doesn't use the constants defined by the libass headers,
but it's still better than exposing those to the user directly.
options: allow selecting the libass shaper

I'm using the word "languages" instead of "scripts" in the manpage, but
I think that's easier to understand with a smaller amount of
descriptions.
vaapi: fix compilation with Libav

The code using FFSWAP was moved from vo_vaapi.c to vaapi.c, which didn't
include libavutil/common.h anymore, just libavutil/avutil.h. The header
avutil.h doesn't include common.h recursively in Libav, so it broke
there.

Add FFSWAP as MPSWAP in mp_common.h (copy pasted from ffmpeg) to make
sure this doesn't happen again. (This kind of stuff happens all too
often, so screw libavutil.)
Fix previous commit

This time it broke because I didn't actually test compiling vo_vaapi.c,
and it was using a macro from mp_image.h, which implicitly assumed
FFALIGN was available. Screw that too, and copy the definition of
ffmpeg's FFALIGN to MP_ALIGN_UP, and move these macros to mp_comnon.h.
Add initial Lua scripting support

This is preliminary. There are still tons of issues, and any aspect
of scripting may change in the future. I decided to merge this
(preliminary) work now because it makes it easier to develop it, not
because it's done. lua.rst is clear enough about it (plus some
sarcasm).

This requires linking to Lua. Lua has no official pkg-config file, but
there are distribution specific .pc files, all with different names.
Adding a non-pkg-config based configure test was considered, but we'd
rather not.

One major complication is that libquvi links against Lua too, and if
the Lua version is different from mpv's, you will get a crash as soon
as libquvi uses Lua. (libquvi by design always runs when a file is
opened.) I would consider this the problem of distros and whoever
builds mpv, but to make things easier for users, we add a terrible
runtime test to the configure script, which probes whether libquvi
will crash. This is disabled when cross-compiling, but in that case
we hope the user knows what he is doing.
Add the on-screen-controller

Signed-off-by: wm4 <wm4@nowhere>
Merge remote-tracking branch 'upstream/master'
Update