Bug 947160 remove MOZ_VP8_ENCODER. r=kinetik

  - Encoder is always built, remove MOZ_VP8_ENCODER.
  - Rename MOZ_VP8* to MOZ_VPX*
  - Enable multi-res encoding via libvpx configure.
Bug 947160 - No need to split encoder sources anymore. r=kinetik
Bug 947954 - vpx_config.c is not used. r=kinetik