BUG: VTKPolyDataMeshIO Write should use signed char for CHAR

Both `VTKPolyDataMeshIO::WritePoints` and `VTKPolyDataMeshIO::WritePointData`
originally casted the `buffer` to plain `char *` for component type `CHAR`
(using `CASE_INVOKE_WITH_COMPONENT_TYPE`). However, `CHAR` indicates that the
component type is _signed_ char, whereas the default `char` might be unsigned.

Follow-up to pull request #5470
commit 70fbb2b "BUG: MINCImageIO should use
`signed char` for `CHAR` and `MI_TYPE_BYTE`"

Author: N-Dekker

Modules/IO/MeshVTK/src/itkVTKPolyDataMeshIO.cxx

@@ -1696,7 +1696,7 @@ VTKPolyDataMeshIO::WriteMeshInformation()
   }                                                                                    \
   case IOComponentEnum::CHAR:                                                          \
   {                                                                                    \
-    function(outputFile, static_cast<char *>(buffer), " char");                        \
+    function(outputFile, static_cast<signed char *>(buffer), " char");                 \
     break;                                                                             \
   }                                                                                    \
   case IOComponentEnum::USHORT:                                                        \