perf: Do not update the feature modified time when changing visibility

This caused needless rebuilds of features and does not change exposed
behavior

Author: manthey

src/feature.js

@@ -733,7 +733,6 @@ var feature = function (arg) {
     }
     if (m_visible !== val || direct) {
       m_visible = val;
-      m_this.modified();
       if (m_layer && m_layer.visible && !m_layer.visible()) {
         val = false;
       }

tests/cases/feature.js

@@ -201,7 +201,7 @@ describe('geo.feature', function () {
       var modTime = feat.timestamp();
       expect(feat.visible(false)).toBe(feat);
       expect(feat.visible()).toBe(false);
-      expect(feat.timestamp()).toBeGreaterThan(modTime);
+      expect(feat.timestamp()).toBe(modTime);
 
       expect(feat.visible(true)).toBe(feat);
       var depFeat = geo.feature({layer: layer, renderer: layer.renderer()});
@@ -210,7 +210,7 @@ describe('geo.feature', function () {
       expect(feat.visible(false)).toBe(feat);
       expect(feat.visible()).toBe(false);
       expect(depFeat.visible()).toBe(false);
-      expect(depFeat.timestamp()).toBeGreaterThan(modTime);
+      expect(depFeat.timestamp()).toBe(modTime);
       feat.dependentFeatures([]);
       expect(feat.visible(true)).toBe(feat);
       expect(depFeat.visible()).toBe(false);