Replaced calculations between "Down Y" and "Format" space with calls to Format::yDownToFormatSpace().
Fixed some bugs when using negative display windows.
Fixed bugs when reformatting one image with a negative format to another.
Fixed the expected output of a test case which was wrong.
Validated the results against the result of a similar operation in other software.
Merge branch 'ReaderBug' of https://github.com/goddardl/gaffer into goddardl-ReaderBug
Beefed up negative display window test a little.
Removed unused variable in Reformat::computeChannelData().