vf-metadata: fix handling of NULL metadata

lavfi would segfault due to a NULL dereference if it was asked for its
metadata and none had been allocated (oops). This happens for libav
which has no concept of filter metadata.
vf_lavfi: reset metadata in reset()

It might have been nice not to do this so that metadata could
accumulate accross seeks, but it seems libavfilter looses its copy
anyway on recreate_graph.
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 the hot-key is pressed, it now inserts the cropdetect filter and
waits 1 second (or a --lua-opts specified duration) before gathering
the cropdetect metadata and inserting the appropriate crop filter. A
second press of the hotkey removes the crop.
TOOLS: better documentation of lua scripts