autoloading videodriver-murdering gstreamer-plugins-vaapi became the default

[v-fox]

https://cgit.freedesktop.org/cgit/?url=gstreamer/www/tree/src/htdocs/releases/1.8/release-notes-1.8.md
https://bugs.freedesktop.org/show_bug.cgi?id=91322

On Mesa at best it's slow, at worst - kernel drm crash.
A way to disable its autoloading is needed, otherwise: load Firefox, open any HTML5 video, like anything on Youtube — get dunked on !

[v-fox]

Seems like it will not be a problem since FF is removing gstreamer: https://bugzilla.mozilla.org/show_bug.cgi?id=1207429#c27
It should be used only as fallback but it may be useful to disable media.gstreamer.enabled option just to make sure.