plugin cache: do not re-write the cache file unless explicitly requested by the user (closes #14519)

The QuickDraw vout is gone since  a long time so this remnant is a NO-OP

We need to know it before MediaCodec is started.

Try a delayed opening only if csd changed.

Allocating and reading 100 MiB at once for a remote file was a bit
excessive IMHO.

If there is a parent input, it cannot go away. There was no point in
reference counting.

This is arguably the only case where the callback made sense.