fix: xmodel glTF dumps dropping COLOR_0 vertex attributes

[michaeloliverx]

I ran into this while working in my Xenon fork, but the root cause turned out to be upstream.

Preserve COLOR_0 in xmodel glTF dumps. The writer was dropping vertex colors from dumped GLBs, causing readers to fall back to default white and breaking flag/cloth deformation masks.

The video shows the original map on the left and an OAT-linked version on the right using the .glb loader for xmodel,bro_tarp_crate_anim from mp_osg_cargo.

iw3_oat_xmodel_bug.mp4

EDIT: You can also observe the issue on stockmaps with flags e.g. mp_showdown

[Laupetin]

Good find, thanks :D I adjusted the code a bit to only dump the color data when there is non-default data, meaning not every vertex has a color value of 1,1,1,1.
Just so that it's maybe easier to work with models that do not require handling a separate color layer on vertices.