Some used 'is_valid()' checks, others not. Validity is already checked in 'unref()',
and 'remove_resource_format_*()' has an ERR_FAIL condition on 'is_null()' already
(which shouldn't happen since we're only unregistering things that we previously
registered.
Also add missing GDCLASS statement in ResourceFormatLoaderVideoStreamGDNative,
missed in #20552 which was last amended before #19501 was merged.
Interface and callback api added for Videodecoder support.
Should be able to construct any format videodecoder using
only the given interface.
GSoC 2018 project.
