ImageWriter : Modify writer to write DisplayWindow
ImageWriter : Modify writer to write DisplayWindow

This commit changes ImageWriter to ensure that, when writing
to image formats that don't support separate display and data
windows, it writes out the display window, padding with black
where necessary.

It also fixes tiled data writing, which was previously broken

boberfly/gaffer,johnhaddon/gaffer,lucienfostier/gaffer,johnhaddon/gaffer,appleseedhq/gaffer,hradec/gaffer,GafferHQ/gaffer,boberfly/gaffer,andrewkaufman/gaffer,appleseedhq/gaffer,boberfly/gaffer,GafferHQ/gaffer,andrewkaufman/gaffer,hradec/gaffer,andrewkaufman/gaffer,ImageEngine/gaffer,hradec/gaffer,ivanimanishi/gaffer,ivanimanishi/gaffer,ImageEngine/gaffer,GafferHQ/gaffer,GafferHQ/gaffer,hradec/gaffer,chippey/gaffer,johnhaddon/gaffer,lucienfostier/gaffer,andrewkaufman/gaffer,boberfly/gaffer,ImageEngine/gaffer,lucienfostier/gaffer,ivanimanishi/gaffer,appleseedhq/gaffer,appleseedhq/gaffer,ImageEngine/gaffer,chippey/gaffer,chippey/gaffer,ivanimanishi/gaffer,chippey/gaffer,hradec/gaffer,johnhaddon/gaffer,lucienfostier/gaffer,johnhaddon/gaffer,andrewkaufman/gaffer,GafferHQ/gaffer