hwcontext: add av_hwdevice_ctx_create_derived_opts

This allows for users who derive devices to set options for the new device context they derive. The main use case of this is to allow users to enable extensions (such as surface drawing extensions) in Vulkan while deriving from the device their frames are on. That way, users don't need to write any initialization code themselves, since the Vulkan spec invalidates mixing instances, physical devices and active devices. Apart from Vulkan, other hwcontexts ignore the opts argument since they don't support options at all (or in VAAPI and OpenCL's case, options are currently only used for device selection, which device_derive overrides).

LYLynne committed 6 years ago