add 1px tolerence in Plots.didMarginChange

... to avoid potential infinite loops when rounding errors
    make auto-margins fail to converge.
    See report (that @etpinard was never able to reproduce) in
    #3561 (comment)

Author: etpinard

src/plots/plots.js

@@ -1911,7 +1911,9 @@ plots.didMarginChange = function(margin0, margin1) {
         var k = marginKeys[i];
         var m0 = margin0[k];
         var m1 = margin1[k];
-        if(!isNumeric(m0) || Math.abs(m1 - m0) > 0) {
+        // use 1px tolerance in case we old/new differ only
+        // by rounding errors, which can lead to infinite loops
+        if(!isNumeric(m0) || Math.abs(m1 - m0) > 1) {
             return true;
         }
     }

test/jasmine/tests/plots_test.js

@@ -988,6 +988,19 @@ describe('Test Plots', function() {
                     plotCallCnt: 1
                 });
             })
+            .then(function() {
+                gd._fullLayout._pushmargin.legend.r.size += 1;
+                return Plots.doAutoMargin(gd);
+            })
+            .then(function() {
+                // see https://github.com/plotly/plotly.js/issues/3561#issuecomment-485953778
+                // for more info
+                _assert('after doAutoMargin() with bigger margins under tolerance', {
+                    r: r0 + 3,
+                    w: w0 - 3,
+                    plotCallCnt: 0
+                });
+            })
             .catch(failTest)
             .then(done);
         });