ImageWriterTest : Add missing offsets
ImageWriterTest : Add missing offsets

I assume this is what the loop on the `offset` variable was intended for, as otherwise it is completely unused, and the `o` node remains at default values throughout.

GafferHQ/gaffer,boberfly/gaffer,hradec/gaffer,lucienfostier/gaffer,ImageEngine/gaffer,hradec/gaffer,andrewkaufman/gaffer,lucienfostier/gaffer,ImageEngine/gaffer,andrewkaufman/gaffer,boberfly/gaffer,johnhaddon/gaffer,andrewkaufman/gaffer,GafferHQ/gaffer,hradec/gaffer,johnhaddon/gaffer,boberfly/gaffer,lucienfostier/gaffer,hradec/gaffer,andrewkaufman/gaffer,lucienfostier/gaffer,ImageEngine/gaffer,andrewkaufman/gaffer,GafferHQ/gaffer,johnhaddon/gaffer,johnhaddon/gaffer,ImageEngine/gaffer,hradec/gaffer,johnhaddon/gaffer,boberfly/gaffer,GafferHQ/gaffer,GafferHQ/gaffer