asset requirements

[mrgreywater]

There are some requirements for the assets such as:

• Minimum 64 guide strands: https://github.com/GPUOpen-Effects/TressFX/blob/master/amd_tressfx/src/TressFXAsset.cpp#L307 (also if the number of guide hairs is not a multiple of 64, some of them will be removed)
• Number of vertices per strand being bigger a power of two bigger than 2: https://github.com/GPUOpen-Effects/TressFX/blob/master/amd_tressfx/src/TressFXAsset.cpp#L285

It would be nice if these requirements could be resolved automatically by resampling the hairs to generate new vertices if necessary, and adding (invisible or duplicate) guide hairs, instead of asserting or crashing.