refactor: rename render 'layers' to 'buckets'

- the name I was looking for for a long time :)
  also less confusion with map layers
- move renderer.elements -> renderer.bucket
- rename RenderBuckets functions
- rename BucketRenderer.layers -> buckets

vtm-web/src/org/oscim/gdx/emu/org/oscim/utils/Tessellator.java

@@ -1,7 +1,7 @@
 package org.oscim.utils;
 
 import org.oscim.core.GeometryBuffer;
-import org.oscim.renderer.elements.VertexData;
+import org.oscim.renderer.bucket.VertexData;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;