Added TODOs to indicate source of TestQuadricLODActor failure

The TestQuadricLODActor test triggers signed overflow,
which is undefined behaviour in C++ and caught
by clang's ubsan:

vtkQuadricClustering.cxx:592:52: runtime error: signed integer overflow: 2560000 * 839 cannot be represented in type 'int'

Also changed several uses of 'int' to be 'vtkIdType',
to reduce chance of overflow and just to better match API declarations.

Moved some variable declarations to their usage site.

Change-Id: Id9b827d73d1da6bfdb9470091baad7d8eb052de2
Merge topic 'signed-overflow' into master

486dc552 Added TODOs to indicate source of TestQuadricLODActor failure