F3D 3.0 crashes with certain VRML files

[mwestphal]

Describe the bug
F3D support reading VRML files, however, since the rework of file loading in 3.0.0, some of these files now crashes F3D

To Reproduce
Steps to reproduce the behavior:

1. Download and extract snowman.zip
2. Open the file using f3d --no-config snowman.wrl
3. crash

Expected behavior
No crash and correct display. Please note display may be incorrect because of #826

Additional context
Caused by the vtkF3DMetaImporter.cxx:241

vtkPolyDataMapper::SafeDownCast(actor->GetMapper());

Which does not check that the mapper exists. However we should investigate if that assumption of always getting a mapper is correct or not and how it should be handled.