Use build_root even if it's deprecated in order to not change the
minimum required meson version.

...to reduce the memory use.

to save some memory on big lists.

instead of using a fixed value.

Convert the video thumbnailer to an external command to avoid potential
crashes or excessive memory growth when genereting thumbnails.

for the metadata reader as well.

It is a synchronous function so there is no need for
a callback.

This allows to have video thumbnails without relying on external
programs.

It was off by 1 pixel.

For example this allows to register an image loader for video/*