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.

test/rendering/ol/reproj/tile.test.js

@@ -12,7 +12,7 @@ describe('ol.rendering.reproj.Tile', function() {
   function testSingleTile(source, targetProjection, targetTileGrid, z, x, y,
     pixelRatio, expectedUrl, expectedRequests, done) {
     const sourceProjection = source.getProjection();
-    const sourceGutter = source.getGutter(sourceProjection);
+    const sourceGutter = source.getGutterForProjection(sourceProjection);
 
     let tilesRequested = 0;