Conditionally render tiles to a separate tile canvas

Because clip geometries are anti-aliased in most browsers, there will be tiny
gaps between tiles. If tiles are rendered to a tile canvas which is then drawn
to the map canvas upon composition, these gaps can be avoided. For rotated
views, it is stil necessary to clip the tile, but in this case a 1-pixel
buffer is used.

This change also brings a huge performance improvement for panning, because
the fully rendered tiles can be reused.

Because of the added cost of using drawImage in addition to replaying the tile
replay group, we fall back to directly drawing to the map canvas when the tile
canvas would be too large, or during interaction/animation when resolution or
rotation change.

src/ol/vectortile.js

@@ -4,6 +4,7 @@ goog.require('ol.Tile');
 goog.require('ol.TileCoord');
 goog.require('ol.TileLoadFunctionType');
 goog.require('ol.TileState');
+goog.require('ol.dom');
 goog.require('ol.proj.Projection');
 
 
@@ -33,6 +34,12 @@ ol.VectorTile =
 
   goog.base(this, tileCoord, state);
 
+  /**
+   * @private
+   * @type {CanvasRenderingContext2D}
+   */
+  this.context_ = ol.dom.createCanvasContext2D();
+
   /**
    * @private
    * @type {ol.format.Feature}
@@ -84,6 +91,14 @@ ol.VectorTile =
 goog.inherits(ol.VectorTile, ol.Tile);
 
 
+/**
+ * @return {CanvasRenderingContext2D}
+ */
+ol.VectorTile.prototype.getContext = function() {
+  return this.context_;
+};
+
+
 /**
  * @inheritDoc
  */