Update the WebP Container Spec.

- Add details about the VP8L chunk support.
- Also add new example bitsteams containing VP8L chunk.
- Add back a section describing the VP8 chunk.
- Restrict some fields to 16 or 24 bits instead of 32 bits.
- Fields whose values are always positive are stored 1-based
instead of 0-based.
- Unknown chunks can only occur at certain places in the file.
- Remove the restriction for some fields to be divisible by 32 or 16.
  Instead they are restricted to be even.
- Add a restriction for (canvas_width * canvas_height) product.
- Add 3 bits for rotation & symmetry in VP8X flags.
- Add some new example layouts.
- Add/clarify some nitty-gritties throughout the doc.
- Use the terms frame/tile more consistently and logically.
- Update related TODOs.

Change-Id: I611c1f58ecc3ee87546ca31bad1609376fad251e

doc/TODO

@@ -3,10 +3,6 @@
 * Determine that normative RFC 2119 terms (MUST, SHOULD, MAY, etc.) are
   truly intended in all cases where capitalized.
 
-* Document hierarchy WRT headings has a flaw, in that topics related to
-  animated WebPs are discussed under subheads of "Single-image WebP
-  Files".
-
 * Several passages could be made clearer.
 
   * Overall edit for scope.  Portions are phrased as an introduction to
@@ -15,10 +11,3 @@
 
   * To wit, suggest s/[spec|specification]/guide/g .  "Spec" can imply a
     standards track; in any case it's too formal for a work in progress.
-
-  * Sections and passages re "multi-image" should likely be suppressed
-    until multi-image drops.
-
-  * Improve the term "Mux-Container".
-
-