COMP: Disable png symbol mangling for easier integration with other project.

This is obviously a hack, VTK should be updated to propose a way
to optionally disable the mangling.
For example: https://github.com/commontk/zlib/commit/66a753054
BUG: undeclared member in vtkTextureTable.

Fixed the bug introduced in commit 'GPU raycast volume rendering now supports textures larger than 1024'