video: add VapourSynth filter bridge

Mainly meant to apply simple VapourSynth filters to video at runtime.
This has various restrictions, which are listed in the manpage.

Additionally, this actually copies video frames when converting frame
references from mpv to VapourSynth, and a second time when going from
VapourSynth to mpv. This is inefficient and could probably be easily
improved. But for now, this is simpler, and in fact I'm not sure if
we even can references VapourSynth frames after the core has been
destroyed.
client API: add mpv_get_wakeup_pipe convenience function

Should make integreating with some event loops easier. Untested.
lua: wrap mpv_get_wakeup_pipe()

Pretty much experimental for issue #661.
lua: make it easier to integrate with foreign event loops

We provide some "official" utility functions for this.
manpage: lua: separate "advanced" functions into a separate section

This will be less confusing.
mp_tags: move generic mp_tags stuff into its own .c/.h files in common/

rename add_metadata to the more genera/descriptive mp_tags_copy_items_from_av_dictionary
m_property: add missing convenience macros for int64 and double subproperties
talloc README: more human readible api reference
vf: add three new functions for sending vfctrls down the filter chain

vf_command_all: send command to all filters
vf_command_by_name: send command to named filter
vf_control_by_label: send command to a filter with given label
vf: auto-label video filters if no label is specified.
vf_lavfi: fix ffmpeg deprecation warning for avfilter_graph_parse

use avfilter_graph_parse_ptr for ffmpeg
vf-metadata: copy AVFrame metadata into vf_lavfi priv

store it as mp_tas and add VFCTRL_GET_METADATA to access it from elsewhere
vf-metadata: add vf-metadata property

This is a read-only property that uses VFCTRL_GET_METADATA
to retrieve mp_tags metadata from a filter specified by label
vf-metadata: lua cropdetect example script
vf-metadata: test script
af_lavfi: fix graph parse deprecation warning
af_volume: fix clang -Wsometimes-uninitialized