Determined experimentally, on various videos and hardware.
On Intel, using less resources in-flight is around 15% faster,
with similar results on Nvidia hardware.
According to Dave Airlie:
> <airlied> but I think ignoring it should be fine, I can't see any
> other way to get the imaeg extents correct for other usage
> <Lynne> what width/height should be used for the images?
> the final presentable dimensions, or the coded dimensions?
> <airlied> if you don't want noise I think the presentable dims
> <airlied> the driver should round up the allocations internally,
> but if you are going to sample from the images then w/h have to be
> the bounds of the image you want
> <airlied> since otherwise there's no way to stop the sampler from
> going outside the edges
Apparently, the alignment values are informative, rather than mandatory,
but the spec's wording makes it sound as if they're mandatory.
