Test for invalid length normals in normal maps

[emackey]

Suggested in KhronosGroup/glTF#2368, this utility could check for invalid texels in normal maps by calculating the length of the normal at each texel, and checking for Z < 0.5. There might need to be some tolerance to allow for poor JPEG encoding, etc. This could be done independently of any mesh geometry, but would require image decoders to be present.