Use negative frame index as frame marker

This PR fixes the canvas tile example (see #639).

The problem was that `ol.DebugTile_#getKey()` falls back to the default `ol.Tile#getKey()` implementation, which uses `goog.getUid(this)` as the key. `goog.getUid` returns numbers that were conflicting with the use of frame indexes as frame markers in the WebGL renderer's LRU texture cache (see #564).

The PR fixes the problem by negating the frame index used a frame marker, thus avoiding the conflict with `goog.getUid`.