Fix link issues with ConstantImplicitBackend and CompositeImplicitBackend
Linker complained that a symbol was redefined when using these classes in a target while also linking against CommonCore: LNK2005 "vtkConstantImplicitBackend" already defined in vtkToImplicitRamerDouglasPeuckerStrategy.obj This was caused by an explicit instanciation without proper extern template declaration.
Fixes #19068 (closed)
Backport: release
Edited by Alexy Pellegrini