Keep move cursor while adjusting imagery offset

(closes #3101)

css/app.css

@@ -2042,6 +2042,17 @@ div.full-screen > button:hover {
     right: -10px;
 }
 
+.nudge-surface {
+   position: absolute;
+   z-index: 5000;
+   left: 0;
+   top: 0;
+   width: 100%;
+   height: 100%;
+   background-color: transparent;
+   cursor: move;
+}
+
 .background-control .nudge.right::after {
     border-top:  5px solid transparent;
     border-bottom:  5px solid transparent;

js/id/ui/background.js

@@ -204,6 +204,10 @@ iD.ui.Background = function(context) {
         function dragOffset() {
             var origin = [d3.event.clientX, d3.event.clientY];
 
+            context.container()
+                .append('div')
+                .attr('class', 'nudge-surface');
+
             d3.select(window)
                 .on('mousemove.offset', function() {
                     var latest = [d3.event.clientX, d3.event.clientY];
@@ -216,6 +220,9 @@ iD.ui.Background = function(context) {
                     nudge(d);
                 })
                 .on('mouseup.offset', function() {
+                    d3.selectAll('.nudge-surface')
+                        .remove();
+
                     d3.select(window)
                         .on('mousemove.offset', null)
                         .on('mouseup.offset', null);