Fix desync by properly clearing mv_grid in the decoder
Fix desync by properly clearing mv_grid in the decoder

The desync happens since dd3fca176f087d8fe72ddf2e972d728ef92d941f which
assumes that the grid is cleared. It was reproducable by encoding 16
frames of parkjoy_1080p at -v 50.

xiphmont/daala,tribouille/daala,mbebenita/daala,kbara/daala,nvoron23/daala,mbebenita/daala,jmvalin/daala,xiphmont/daala,ycho/daala,tribouille/daala,HeadhunterXamd/daala,vr000m/daala,iankronquist/daala,ycho/daala,nvoron23/daala,tribouille/daala,HeadhunterXamd/daala,jmvalin/daala,vr000m/daala,tribouille/daala,ycho/daala,felipebetancur/daala,iankronquist/daala,iankronquist/daala,xiph/daala,iankronquist/daala,felipebetancur/daala,kustom666/daala,ycho/daala,luctrudeau/daala,xiph/daala,luctrudeau/daala,ascent12/daala,KyleSiefring/daala,KyleSiefring/daala,HeadhunterXamd/daala,nvoron23/daala,kodabb/daala,xiphmont/daala,kodabb/daala,kbara/daala,xiphmont/daala,kustom666/daala,HeadhunterXamd/daala,nvoron23/daala,jmvalin/daala,ascent12/daala,KyleSiefring/daala,xiph/daala,ycho/daala,kodabb/daala,xiphmont/daala,mbebenita/daala,kustom666/daala,luctrudeau/daala,kbara/daala,HeadhunterXamd/daala,kustom666/daala,iankronquist/daala,jmvalin/daala,kbara/daala,luctrudeau/daala,felipebetancur/daala,nvoron23/daala,kodabb/daala,ascent12/daala,kodabb/daala,xiph/daala,mbebenita/daala,ascent12/daala,mbebenita/daala,felipebetancur/daala,KyleSiefring/daala,felipebetancur/daala,tribouille/daala,jmvalin/daala,vr000m/daala,kustom666/daala,xiph/daala,ascent12/daala,vr000m/daala,luctrudeau/daala,vr000m/daala,KyleSiefring/daala