thumbnails: png output

[gioelecerati]

• Use png as output for thumbnails
• Remove q:v quality flag from thumbnails generation

q:v was added previously as an attempt to fix thumbnail quality - however reports of low quality thumbnails were due to some specific HDR videos. Having png as output fixes the issue

[thomshutt]

Could we reduce the default resolution a little bit too please? I think the one now is excessive and it'll hopefully keep filesizes under control after the move to PNG