Support for ASTC compressed textures wrapped in KTX files

Support for ASTC compressed textures wrapped in KTX files

ASTC is supported by OpenGL ES 3.1 and above.
To build this patch with ASTC support, a GLES 31 context must be specified.

To build for desktop with configure, the gles version must be set to 31.
Here is an example configure line:
CXXFLAGS="-g -O0 -Wno-unused-local-typedefs" CXX="ccache g++" ./configure --prefix=$DESKTOP_PREFIX --enable-debug=yes --enable-profile=UBUNTU --enable-gles=31

With GBS, add the following to your gbs build line:
--define "%target_gles_version 31"

Change-Id: I376e55094e408c1b9081a3e86b81e7b3d64633bd