#1415 Added KTX of ETC1 support to GLES2

[oiram]

PKM is a simple file format for single compressed images. If you generate Mipmaps, multiple PKM files are created.
KTX is a Khronos Group standard compression format that provides a container for images. If you generate Mipmaps, a single KTX file is generated.

Wolfmanfx wrote the original codec for add PKM support:
http://www.ogre3d.org/forums/viewtopic.php?f=21&t=70645#p461088

I add KTX support base on it, and it was tested that works fine.

[oiram]