Tile returning array of <symbol>s (#33)

* Mapping can now return an array of strings or VDOM tiles, each with
  their own `z-index`.  First non-null tile controls `viewBox`.
* `chess` examples save 15-35% by avoiding duplication of background tiles

README.md

@@ -211,15 +211,23 @@ or return value of a function) should be specified as one of the following:
 6. An empty string, short for the empty symbol `<symbol viewBox="0 0 0 0"/>`.
 7. `undefined` or `null`, indicating that this mapping doesn't define a tile
    for this tile name (and the next mapping should be checked).
-8. Another mapping (JavaScript object, `Map` object, or function) that gets
+8. An `Array` of tiles of any of the above formats (or more `Array`s, which
+   get flattened), meaning to stack multiple tiles on top of each other,
+   where the first non-null tile defines the `viewBox` and size
+   (but all can influence the [`overflowBox`](#overflow-and-bounding-box)).
+   Use [`z-index`](#z-index-stacking-order-of-tiles)
+   to control stacking order.
+   Null items in the array get ignored, and an empty array acts like `null`
+   (this mapping does not define a tile for this tile name).
+9. Another mapping (JavaScript object, `Map` object, or function) that gets
    recursively evaluated as described above (with the same tile name and
    context).  For example, a top-level JavaScript object could map some tile
    names to functions (when they need to be dynamic); or a top-level function
    could return different mappings depending on context.
-9. One of the tiles above wrapped in a call to `svgtiler.static`, e.g.,
-   `svgtiler.static(<symbol/>)`.  This wrapper tells SVG Tiler that the tile
-   mapping is always the same for this tile name, and does not depend on
-   `Context` (e.g. adjacent tiles), enabling SVG Tiler to do more caching.
+10. One of the tiles above wrapped in a call to `svgtiler.static`, e.g.,
+    `svgtiler.static(<symbol/>)`.  This wrapper tells SVG Tiler that the tile
+    mapping is always the same for this tile name, and does not depend on
+    `Context` (e.g. adjacent tiles), enabling SVG Tiler to do more caching.
 
 If you need to use a `<marker>`, `<filter>`, gradient, or other element
 intended for `<defs>`, call `svgtiler.def(tag)`, where `tag`