[FS25] Merged mesh for FS25

[LKAMinco]

Merged mesh in FS25 needs to merge materials too, but exporter can't handle it, and export just materials from root.

Update
If all objects from merged group do not contain all materials from MG, the GE will crash.
If non root object contains all materials it will open GE but it won't render non root objects

Update 2
It will not merge child objects

[StjerneIdioten]

This would break FS22 right? Unless we somehow make support for both cases

[LKAMinco]

Actually, i do not know, it is just support for multiple materials, which should not be a problem for fs22 too