Add KTX file format support + Zipped variant

When working with ETC1, you do not have an easy way to load compressed texture with all its mipmaps. Khronos have specified a file format for this (and other things) here : https://www.khronos.org/opengles/sdk/tools/KTX/file_format_spec/ the Mali ETC1 compression tool directly create them (http://malideveloper.arm.com/develop-for-mali/tools/asset-creation/mali-gpu-texture-compression-tool/).

This pull request add KTX file format support to texture (including ETC1 on the fly unpacking as it exists for ETC1 textures). 

It also provides a gzip variant (called 'ZKTX') since like for ETC1, KTX format is not compact and is better gzipped. To Create a zipped variant, just run the KTX against GZipOutputSTream, adding the size of the file as an int at beginning (like ETC1). I can provide code if you wish.

It has been tested on desktop and Android. Not on GWT but ECT1 is already lacking on GWT. I do not have the possibility to test on iOS. I left the code for 1D and 3D texture but since LibGdx do not support these either, this can be removed on demand.