Layered animation support

Update the FBX file format and exporter
to support layered skeletal animations. This
includes the following fixes:

1. The parent index for the first bone was being
    output incorrectly.

2. Output the local bone transform for each bone;
    we can use this to determine if a given skeletal
    animation is influencing the bone by comparing it
    the animation’s local animation transform.

3. For each skeletal animation, output the local transform
    *and* the concatenated transform.

4. Instead of outputting the concatenated transform
    multiplied by the inverse matrix (to go from bone
    space to bone space), instead just output the
    concatenated transform (which goes from bone space
    to model space). This reduces the operations required
    in the renderer. When rendering layered animations, we
    will use local transforms instead of concatenated
    transforms, so preserving the bone-space to bone-
    space matrices isn’t necessary.

Author: radvani