gc.p_sys can now be NULL, if the picture has no planes.

The X server supplies a large but nevertheless finite range of XID for
the X client to use. So lets reuse our handles.

psz_realhost is an alias to psz_socks when using socks.

fixes #8942. Default value for avcodec-hw is none, but NULL was saved, so
hardware decoding got enabled unintentionally.