Add support for KTX / ZKTX file format

This is a follow up to my first dirty PR : https://github.com/libgdx/libgdx/pull/2418

This time it comes with clean rebased commit (no newline bugs and such, guess this is the way we learn...).
On the feature side ;
- cleaned up the cube map (which in turns was buggy),
- add a better test with cubemap, and ETC1 with separate alpha example,
- add a file processor to generate KTX, ZKTX, packing, mipmaps, and alpha atlas (a bit quick and dirty on this).

This required a change outside the initial scope on CubemapAttribute to handle other GLTexture than Cubemap.java (did not find a clean way to put KTX packed cubemap in the current implementation and did not want to make change that would not be retro compatible).

Now on the what is missing ; 
- a wiki page on ETC1 compression, KTX/ZKTX/ETC1 file formats, cubemap and alpha support
- a note in the changelog with a link to the wiki page
GL_`GL_TEXTURE_CUBE_MAP` isn't a valid value for `target`. See https://www.khronos.org/opengles/sdk/docs/man/xhtml/glTexImage2D.xml
> Specifies the target texture of the active texture unit. Must be GL_TEXTURE_2D, GL_TEXTURE_CUBE_MAP_POSITIVE_X, GL_TEXTURE_CUBE_MAP_NEGATIVE_X, GL_TEXTURE_CUBE_MAP_POSITIVE_Y, GL_TEXTURE_CUBE_MAP_NEGATIVE_Y, GL_TEXTURE_CUBE_MAP_POSITIVE_Z, or GL_TEXTURE_CUBE_MAP_NEGATIVE_Z.