dump_ssim: Make aspect ratio mismatch an error.
dump_ssim: Make aspect ratio mismatch an error.

Upgrading it from a warning.

We scale the Gaussian window according to the pixel aspect ratio in
 order to match the shape and size of the window used by the
 fixed-size test images in the original paper.
If the streams do not have the same aspect ratio, then there is no
 correct thing to do.

ycho/daala,ycho/daala,xiph/daala,ycho/daala,xiph/daala,xiph/daala,xiph/daala,xiph/daala,ycho/daala,ycho/daala