avio: Copy URLContext generic options into child URLContexts

avio: Copy URLContext generic options into child URLContexts

Since all URLContexts have the same AVOptions, such AVOptions
will be applied on the outermost context only and removed from the
dict, while they probably make sense on all contexts.

This makes sure that rw_timeout gets propagated to the innermost
URLContext (to make sure it gets passed to the tcp protocol, when
opening a http connection for instance).

Alternatively, such matching options would be kept in the dict
and only removed after the ffurl_connect call.

Signed-off-by: Martin Storsjö <martin@martin.st>