Need opposite-endian test file(s)

[MarkCallow]

In order to test the byte-swapping code and avoid problems like issue #25 we need an opposite-endian test KTX file. In practical terms that means a big-endian file since all my test machines are little-endian.

[sevmeyer]

As a note, I have recently created some big-endian test files with various type sizes, see endian_big_*.ktx:

https://github.com/sevmeyer/ktxtest/tree/master/ktx/valid/format

They are CC0, you are free to use them if they meet your requirements.