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') wince like for ETC1, KTX format is not compact and is better gzipped.

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.