Expose original getGutter

TileWMS objects take a gutter option but do not have a public getter
for it. This makes it convoluted for user code to recreate the object
(ex: in the case of serialization/deserialization).

- the getGutterInternal() method is renamed to getGutter;
- the getGutter(projection) method is renamed to getGutterForProjection,
  which is also more explicit.

The getGutter method was not API and is only used by the renderer.

src/ol/source/TileWMS.js

@@ -214,7 +214,7 @@ class TileWMS extends TileImage {
   /**
    * @inheritDoc
    */
-  getGutterInternal() {
+  getGutter() {
     return this.gutter_;
   }