This uses strtof rather than stringstream and provides a common location
that can be leveraged by the upcoming JNI bindings.
Note that this new method lives in KtxBundle rather than KtxUtility. The
latter creates Filament textures and therefore does not get built into
libimage.
KtxBundle is basically an in-memory representation of a KTX file.
Our primary motivation for introducing this class is to have a
structured container for block-compressed textures.
Note that this class is defined in libimage rather than libimageio.
There are several reasons for this:
1. The texel data in a KtxBundle is not meant to be decoded by the CPU.
2. It has no dependendencies on any libraries (stb_image tinyexr etc).
3. We wish to use it with WebGL, which needs to avoid using imageio.
