When using the pre-rendered png symbolics it seems that we're off a
tiny bit in a few of the pixels on the antialiased borders of a
stroke. To fix this we switch the icon to media-playback-stop-symbolic
which has no such antialiased borders.
I don't quite understand why the pixels are off, this needs more
research.
https://bugzilla.gnome.org/show_bug.cgi?id=734668