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.
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.