Bug 929398 - Enable vorbis even if webm is disabled. r=ted

We enable either vorbis or tremor, depending on the platform,
if webm is enabled. Both the webm and the ogg media formats
require vorbis support, so the build would fail in the media
playback code if just --disable-webm was passed. Instead enable
vorbis or tremor if either is enabled.
Bug 928530 - Add guards for invalid media switch combinations. r=ted