etc/mpv.desktop: add image/gif

[kirasok]

This PR adds image/gif to the list of mime types in XDG Desktop entry that mpv can open

[sfan5]

I think the lack of image formats is intentional as mpv is primarily not an image viewer. However almost all GIF files you encounter these days are actually animated (not much point still using the format otherwise).
So sounds fine.

[kasper93]

not much point still using the format.

FTFY

I don't mind the patch, but I think mpv is very bad viewer for short animations. Especially with default config. It doesn't loop, and even of you somehow set it to loop it is not smooth and drops frames on transition.