Don't set ortho slice to (0,0,0) when no transform

[nanoant]

When file does not specify origin or/and voxel size then transform matrix is identity. However when loading such file we try to set initial slice position to (0,0,0) regardless there is no coordinate system given. In such case we land in the top left corner, which is a bit confusing. This change checks if transform is identify and then keeps initial slice position at the center of image space.

This makes MRIcroGL also in par with MacOS MRIcro.

[neurolabusc]

Thanks. Hopefully all images will have a valid spatial transform in the header SForm or Form, but without this it makes sense to assume the origin is the center of the volume.