Divide by 0 on pdf images

[lucasderraugh]

For PDF images, -[GIImageDiffView imageSizeWithoutLoadingFromPath:] fails to calculate intrinsic sizes and so CGImageSourceCopyPropertiesAtIndex(imageSource, 0, NULL); yields an empty dictionary.

-[GIImageDiffView desiredImageFrame:] ends up using these values of a {0, 0} frame and dividing by 0 results in a NaN exception.

Before 1.3 ships to the stable channel, we need to handle this case by defaulting to a default value or finding a quick way to calculate in the PDF case.