Fix manta view crash on scene change.

Thanks Takuya Oshima for the patch.

As described in http://markmail.org/message/znrqge6hutq44xjr
This fix addresses the fact that vtk allocates manta triangles in a block.
When the scene changes, manta later deallocates them individually, which causes
the crash.

Change-Id: I3092e5705593da7e27e5cb74620929d086ed09b5
Merge topic '0013760-mantaview-crash' into next

926ef98 Fix manta view crash on scene change.