VPP: Add the option to disable mpeg2 decoding

(big pain on haswell with interlaced content)

I keep it enabled by default, cause SNB and IVB are fine with it. To actually use it (e.g. disable mpeg-2 hw decoding), create

/storage/.xbmc/userdata/advancedsettings.xml with

```
<advancedsettings>
  <video>
    <allowmpeg2vaapi>false</allowmpeg2vaapi>
  </video>
</advancedsettings>
```

I hope that issue gets solved upstream or wip is coming along, so that we can drop that later. For now it's a showstopper on haswell rigs.

Edit: Upstream issue: https://bugs.freedesktop.org/show_bug.cgi?id=72522