--ignore-mips does not apply to array textures

[walbourn]

The --ignore-mips flag add in #484 is intended as a 'header hack' to treats the mipLevel as if it were 1 in DDS files. This is a workaround for some corrupt files. This doesn't work on cubemaps, arrays, or cubemap arrays because of how miplevels are laid out in those files.

This change just makes --ignore-mips only work on arraySize=1 textures since modifying the mipLevel count is not correct for array textures with mips. For array textures/cubemaps, the flag is ignored.

You can remove mips in general by copying out the top-level mips to a new image. The texconv tool will do this with -m 1.