[Tracker] Video thumbnailing

[Sam Thursfield <ssssam gmail com>]

Hi all,

It would be nice if Tracker could provide video thumbnails, in the
same way that it currently extracts album art, for media centre apps
etc. to use. Since we have to open video files anyway to get their
tags and stream info, it makes sense to reuse that pipeline for
thumbnailing - some video rips also have an obvious
folder.jpg/poster.jpg image that we can identify and use instead.

I propose storing them according to the media art spec[2] (I'm sure it
can be easily extended for videos), and we could also generate small
thumbnails following the thumbnail spec[3] to save Nautilus the job.

Does anyone have comments on whether this is a good/bad idea? I worry
that tagreadbin may negate some of the benefits of thumbnailing
in-process, although we will still avoid detected the file format
twice and I think it generally makes sense for tracker to be doing the
thumbnailing instead of every app that wants video thumbnails making
their own private ones (this is what media-explorer does at the
moment, for example).

Thanks
Sam

[1] https://live.gnome.org/ThumbnailerSpec
[2] https://live.gnome.org/MediaArtStorageSpec
[3] https://people.freedesktop.org/~vuntz/thumbnail-spec-cache/