sysutils/plasma5-systemsettings and x11-wm/plasma5-kwin: Fix desktop effects previews.

An obscure gstreamer bug [1] prevents .ogv videos from being played via curl
plugin. Workaround this by re-encoding Theora videos into WebM
(suggested at KDE Devel Matrix channel) and store it on my site for the
time being.

[1] https://gitlab.freedesktop.org/gstreamer/gstreamer/-/issues/2026

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