This follows up 96e1f1dfa5 which converted --gpu-context, and has the
same advantages as listed there.
Unlike with --gpu-context auto can be used anywhere in the list, e.g.
--gpu-api=d3d11,auto works.
I wanted to use the list of GPU contexts as the description in
get_type_desc(), but there is no talloc context to allocate it to, so I
set a print_help_list to print them. The APIs go before the contexts so
that etc/_mpv.zsh doesn't try to complete the contexts.
Guido Cella
dc523b137f