This allows clients to provide their own asynchronous texture decoders
for various mime-typed images. This is a plug-in component for gltfio,
in some ways similar to MaterialProvider.
There are two motivations for this: to decouple gltfio from STB and
to make it easier to integrate support for BasisU textures.
This also has the side effect of simplifying ResourceLoader, since the
texture decoding jobs have been moved out.
As part of this work, I made the "stb" CMake target into a traditional
static library. Previously we had several files called `Image.cpp`
whose sole purpose was to enable STB_IMAGE_IMPLEMENTATION.
