encoder: minor VPx naming/usage clean up

- s/vp8/vpx/
- s/VPx/VpX/
- Add VPX_CODEC_DISABLE_COMPAT and VPX_DISABLE_CTRL_TYPECHECKS; the
  latter allows the collapse of the verbose mess in my
  vpx_codec_control wrapper function.

Change-Id: I174b8c503e1e5643fdbaef6efb3cbe1f65c1eee6