Theora and WebM video streams were mistakenly imported with a ResourceImporter,
but those imported ogvstr and webmstr were simply links to the local resource.
While that works fine in the editor, it no longer works when exporting a game
as the "source" ogv and webm files are ommitted and only the ogvstr and webmstr
references were exported.
As discussed with @reduz, it doesn't make sense to import videos, as we only
intend to play them back and not modify them/access their raw data. As such we
use a ResourceFormatLoader instead of an importer, to load the file on the fly.
ogv and webm files linked to this loader are now considered as resources, and
thus exported.
Note: The Theora and WebM loaders lack any kind of validity check beyond the
existence of the target file, but it was already the case with the importer.
Better checks and error reports could be added, but those loaders will eventually
be obsoleted by GDNative plugins anyway.
Fixes#14954.
