Test for invalid length normals in normal maps

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.

Start with the proposal in KhronosGroup/glTF#2368 and identify how image decoders could be provided for this validator. Define completion as checking normal-map texels for invalid normal lengths and Z values below 0.5 while accounting for possible JPEG encoding tolerance.

Written by the indexing model from the issue text.