feat(geo): support min/max scale limits

fixes plotly#5191

Author: mojoaxel

src/components/modebar/buttons.js

@@ -548,8 +548,18 @@ function handleGeo(gd, ev) {
 
         if(attr === 'zoom') {
             var scale = geoLayout.projection.scale;
+            var minScale = geoLayout.projection.minScale;
+            var maxScale = geoLayout.projection.maxScale;
+
             var newScale = (val === 'in') ? 2 * scale : 0.5 * scale;
 
+            // make sure the scale is within the min/max bounds
+            if(newScale > maxScale) {
+                newScale = maxScale;
+            } else if(newScale < minScale) {
+                newScale = minScale;
+            }
+
             Registry.call('_guiRelayout', gd, id + '.projection.scale', newScale);
         }
     }