Add PLY import/export support to meshconvert

[walbourn]

Common portable asset format .PLY.

https://en.wikipedia.org/wiki/PLY_(file_format)

http://paulbourke.net/dataformats/ply/

Public domain implementation: https://github.com/ddiakopoulos/tinyply

Source asset: http://graphics.stanford.edu/data/3Dscanrep/

[walbourn]

Community PR for adding PLY to UVAtlas to use for reference: microsoft/UVAtlas#70

[walbourn]

The support needs to be opt-in and make use of tinyply via VCPKG instead of including a copy of the OSS source files which is hard to maintain over time.

https://vcpkg.io/en/package/tinyply