Fix RGB ZCT coordinate calculation

[melissalinkert]

As reported by @sbesson, a simple conversion with the --rgb flag logs numerous IllegalArgumentExceptions indicating that the ZCT coordinates could not be calculated.

Without this PR, the build passes, indicating that the unit tests that use the --rgb test also pass. However, examining the test output in build/reports/tests/test/classes/ will show similar IllegalArgumentExceptions in the Standard error section of the test report. The ZCT coordinate calculation is only requested in https://github.com/glencoesoftware/raw2ometiff/blob/master/src/main/java/com/glencoesoftware/pyramid/PyramidFromDirectoryWriter.java#L1537, to get the correct plane index for progress notifications. This happens after the tile has already been written; effectively, it is just the progress notification that is failing. This explains why the test does not fail; the actual conversion does in fact succeed and the output file is valid and readable.

With this PR, the IllegalArgumentExceptions should no longer appear in the test report or conversion output. The actual output OME-TIFF with and without this PR should be unchanged.

[sbesson]

I was originally concerned this would affect the writing but this would be primarily impacting the notification.

Confirmed that with this PR included, both raw2ometiff --rgb -p against a RGB image as well as raw2ometiff -p against a multi-dimensional image work as expected including the progress bar.