Disable texture loading and thumbnails in headless mode #3401

Found by code coverage test in #3337, split off into new item.

WW3D already has switches to turn off texture loading (WW3D::Enable_Texturing) and thumbnail generation (WW3D::Set_Thumbnail_Enabled), but headless mode never uses them.

Set_Thumbnail_Enabled(false) is only called inside the non-headless WW3D::Init block in W3DDisplay::init, and Enable_Texturing(false) is never called at startup. As a result, every texture created in headless still sets itself up and queues thumbnail and background load requests.