Add text background rendering and text padding

src/ol/render/canvas.js

@@ -77,6 +77,13 @@ ol.render.canvas.defaultTextAlign = 'center';
 ol.render.canvas.defaultTextBaseline = 'middle';
 
 
+/**
+ * @const
+ * @type {Array.<number>}
+ */
+ol.render.canvas.defaultPadding = [0, 0, 0, 0];
+
+
 /**
  * @const
  * @type {number}