Fixed threading issue in GafferImage::Display node.
Fixed threading issue in GafferImage::Display node.

Access to the tiles in the display driver is now protected by a mutex.

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