vtkTIFFReader: fix invalid colormap reads
For multipage tiffs, vtkTIFFReader ended up using obsolete colormap since it never refreshed the pointers after a new page was read. This was caught by asan/valgrind. Fixed that.
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For multipage tiffs, vtkTIFFReader ended up using obsolete colormap since it never refreshed the pointers after a new page was read. This was caught by asan/valgrind. Fixed that.