ChannelMerger isInterleaved() can return true

Added a test to ChannelMerger.isInterleaved() - if canMerge() returned
false, ChannelMerger is effectively "disabled" so the isInterleaved()
call is passed to its delegate Reader.
openThumbBytes interleaves return bytes as needed

When merging channels for a resultant byte[], FormatTools.openThumbBytes
now interleaves the channel arrays if reader.isInterleaved() returns
true.

This should fix interleaved image thumbnails displaying incorrectly.
Merge pull request #396 from hinerm/Interleaved-Fixes

Interleaved fixes