[OpenMesh] Patch: PLY face color I/O support

Please find attached a patch for PLY face color support in the PLY reader and writer, including unit testing. The .ply file should be added to src/UnitTests/TestFiles. Happily, this patch is enough to enable color face support from PLYs in OpenFlipper as well. I did not bother to update PLYWriter::binary_size() since that method is already incorrect (it must be unused, or not matter). Also attached is a small patch to FindGoogleTest.cmake for Fedora, etc. compatibility. Steve