ResourceFormatLoader and ResourceFormatSaver are meant to be overridden
to add support for different formats in ResourceLoader and ResourceSaver.
Those should be exposed as they can be overridden in plugins.
On the other hand, all predefined subclasses of those two base classes
are only meant to register support for new file and resource types, but
should not and cannot be used directly from script, so they should not
be exposed.
Also unexposed ResourceImporterOGGVorbis (and thus its base class
ResourceImporter) which are editor-only.
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.
