Recursive expansion in output template

[po5]

Continuation of the work in #92 The original PR would use the first fallback value for all properties of the same name, e.g. %{someproperty:fallbackone} %{otherproperty:fallbacktwo} %{someproperty:fallbackthree} would result in fallbackone fallbacktwo fallbackone. This is fixed.

This supports recursive expansion and comparisons like %{?video-frame-info/picture-type==I:keyframe${estimated-frame-number:X}}. Didn't use expand-text as it doesn't support %{} and because of other annoyances I touch on later.

It has the same limitations as --screenshot-template, a closing curly bracket will close all previous curly brackets, regardless of nesting. This means that instead of %{!fps==22:${height}p} you have to write %{!fps==22:${height}}%{!fps==22:p}. If mpv logic ever changes to fix this, we should follow suit. The only difference with mpv logic currently is that we don't close brackets for the user. With --screenshot-template the above example would be %{!fps==22:${height}%{!fps==22:p} (notice the confusing uneven brackets). As a result, some things like %{!title==${media-title}:untitled} cannot be expressed at all.

[po5]

Something like this?

    -- Template string for the output file
    -- %f - Filename, with extension
    -- %F - Filename, without extension
    -- %T - Media title, if it exists, or filename, with extension (useful for some streams, such as YouTube).
    -- %s, %e - Start and end time, with milliseconds
    -- %S, %E - Start and end time, without milliseconds
    -- %M - "-audio", if audio is enabled, empty otherwise
    -- %R - "-(height)p", where height is the video's height, or scale_height, if it's enabled.
    -- More specifiers are supported, see https://mpv.io/manual/master/#options-screenshot-template
    -- Property expansion is supported (with %{} at top level, ${} when nested), see https://mpv.io/manual/master/#property-expansion
    output_template = "%F-[%s-%e]%M",

[ekisu]

Yeah, that's really good

[ekisu]

Thanks a lot!