Act as a thumbfast provider

[po5]

When the patched OSC is renamed to thumbfast.lua, it will expose a thumbfast-compatible API. The script returns and doesn't act as an OSC. This allows the thumbnailer to work with any UIs that implement this less invasive API: https://github.com/po5/thumbfast#ui-support Positioning and decorations are left up to each client script. Renaming is required as all scripts use script-message-to thumbfast, which mpv forwards only to the specified script. Of course it still works in its normal non-thumbfast mode.

Closes #1

Hoping this will spark renewed interest and we can implement each other's fixes to improve thumbnails. I am the maintainer of thumbfast, if that wasn't obvious.