Support texture coordinates when exporting a PLY

This MR adds support for texture coordinates when exporting a binary PLY, following the same property and layout as the ASCII version (with support for the texture name).

Current behavior: When exporting a mesh to a PLY file, texture coordinates are only exported if the file is saved as ASCII, not as binary. This introduces a discrepancy when saving to different types of PLY: in the general Mesh::save() function, the universal flag toggles from a binary to an ASCII export. It is disabled by default, meaning that a 'default' export will discard the UV information.

Expected behavior: Disabling universal should not cause the UV information to be discarded from the saved file.