DisplayTest : Refactor to provide public `sendImage()` method
DisplayTest : Refactor to provide public `sendImage()` method

This will be used to test the Catalogue, which will use Display nodes internally.

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