The KTX (Khronos texture) file format specifies a simple format for
storing texture miptrees. This patch adds utility functions for reading
and inspecting KTX files, iterating over the contained images, and easily
loading the miptree with glTexImage().

A future test (oes_compressed_etc1_rgb8_test-miptree) will use this to
load an ETC1 miptree and a reference RGB miptree.

Signed-off-by: Chad Versace <chad.versace@linux.intel.com>