TOOLS: improve autocrop.lua

It now inserts no filters and does nothing until the hot-key is pressed.
This makes it more suitable to be put in ~/.mpv/lua.

When they hot-key is pressed, it now removes any existing crop filter and
inserts the cropdetect filter. It then samples the whole file to determine
crop parameters and uses them to insert the new crop filter. The cropdetect
filter is then removed and the original file position restored.