Modified the converter to add a root tranformation node to make all models Y_UP.
Added some description in the comments.
Merge remote-tracking branch 'khronos-gltf/dev-7' into unify-up
Upgrading OpenCOLLADA to the latest.
Fixed unique id generation for the Y up transformation node.
Merged in dev-7
Merge pull request #293 from tfili/unify-up

Adds a transform node to make all models in the Y up reference frame.
Merge remote-tracking branch 'khronos-gltf/dev-7' into libpng
Update CMakeLists to handle libpng better. We now warn if not present and disable functionality. This allows it to work on windows now.