Textures: Add CLI option to control appearance mode (textures or materials)

[bertt]

Currently, pg2b3dm decides automatically whether textures or materials are used
when generating 3D Tiles. In some cases, textured output can result in tiles
that are too complex and negatively affect loading performance in viewers.

Users should be able to explicitly control whether textures or materials
are preferred during generation.

Proposal:

--appearance-mode <textures|materials> (default materials)