Bug#731486: gimp-dds: Unable to save multiple layers as mipmaps

[Tony Houghton]

I've worked out that the problem stems from my image being non-square.
When gimp created mipmaps it made levels/layers from 256x128 through 2x1
but the final layer was 1x1. I looked at the source and found that when
exporting existing layers as mipmaps it validates the layers by looking
for a chain with each successive layer being half the size of the
previous, using a bitshift for a fast divide by 2, so when looking for
the next layer after 2x1 it would try to find 1x0 and fail.