ShellRecorder: improve the default pipeline

The default pipeline was fairly aggressive about quality, and could
be too expensive for some computers. Decrease the quality setting for
the vp8 codec from 10 to 8, and increase the speed setting from 2 to 6.

(Basically, quality affects the visual fidelity of the end result, while
speed affects how much CPU the encoder uses to get a high compression
ratio at that quality level.)

Remove videorate from the pipeline, since the GStreamer VP8 encoder can
handle variable-framerate streams. This means that we won't spend CPU
encoding duplicate frames added by videorate.

https://bugzilla.gnome.org/show_bug.cgi?id=669066

data/org.gnome.shell.gschema.xml.in

@@ -127,7 +127,7 @@
         take care of its own output - this might be used to send the output
         to an icecast server via shout2send or similar. When unset or set
         to an empty value, the default pipeline will be used. This is currently
-        'videorate ! vp8enc quality=10 speed=2 threads=%T ! queue ! webmmux'
+        'vp8enc quality=8 speed=6 threads=%T ! queue ! webmmux'
         and records to WEBM using the VP8 codec. %T is used as a placeholder
         for a guess at the optimal thread count on the system.
       </_description>