Fix #31176: Collada model shows weird textures in editmode

Issue with multimaterial meshes and mface to mpoly conversion being before assignment of materials (which is done on meshobject instancing?).
Added explicit bmesh conversion to MeshImporter which is called from DocumentImporter::import.

fix #30440, Collada import: no faces/uv after bmesh merge

Seems too simple, just missing call to convert_mfaces_to_mpolys.

Collada export: bone parenting is actually against the tail.

Don't know the reasoning for having it be against the tail and not the head.

Collada export: fix no polylist after bmesh-merge

Just needed a call to BKE_mesh_tessface_ensure when exporting geometries.

Fix #30230, Crash after importing collada file

On collada import a -1 layer_index from CustomData_get_layer_index (CD_MTFACE) was used.
This then managed to overwrite last mface's data with uv-set's name so at least it gave a clue where to look for :]

Fix rest of #27022, collada export: add bone parenting of objects

- SceneExporter collects a list of child-objects for armature-object and passes it onto ArmatureExporter
- SceneExporter's writeNodes is then called from ArmatureExporter for matching child-objects for bone.
- ArmatureExporter removes written child-objects from list, objects not exported as being bone parented are exported as direct children of the armature-node.
- Should play nice with current Second Life-compatibility.

A nicer implementation would require some design changes, will have to wait.

Collada export: changes to joints/weights in skincontroller

- Collecting joints/weights in one place, it's easier to exclude zero weights or vertexgroups with no matching bone than trying to match same logic in multiple places.
- Still not exporting -1 joints for vertices without weights, but also not outputting -1 joint + weight for each vertexgroup without a matching bone.
- The exported weights are now normalized.

Last I tested this patch stopped 3ds Max crashing on import of file from #29465 (opencollada / internal .dae).

Minor change collada export; don't even run the parentinv-test when ob->parent is NULL

collada export: Don't write parentinverse if ob->parent is NULL