BUG: ImageAccumulate overflows for large images

If, in a large image, the count for a bin exceeds the max for an
integer, the count overflows and reports the wrong value. This patch
changes the output of ImageAccumulate to an image of vtkIdType.

The patch include a test. To truly validate a configuration, the dimensions
for the test should be set to 2048. For routine testing a small
dimension is recommended.

This bug was uncovered and analyzed by Gib Bogle while trying to use
ImageAccumulate on very large tiff images.

Change-Id: I284bf503e59461acf2c2d1ffb340daa4979582c2
Merge topic 'ImageAccumulateBinOverflow' into master

a9527207 BUG: ImageAccumulate overflows for large images