gl: Need KTX texture parser

Submitted by Julien Isorce @cap

Link to original bug (#723768)

Description

gst-launch-1.0 filesrc location=texture.ktx ! glktxparse ! glimagesink

See example1 here http://www.khronos.org/opengles/sdk/tools/KTX/ (ktxLoadTextureN)

Could inherit from GstBaseParse.

Need to figure out how to manage with GstVideoOverlayCompositionMeta