libvpx: Pull from upstream

source/libvpx/third_party/nestegg/TODO

-- Document when read, seek, tell callbacks are used.
-- Add an automated testsuite.
-- Test (and fix, if necessary) support for unknown sizes.
-- Test (and fix, if necessary) support for large files.
-- Read past unknown elements rather than seeking.
-- Try to handle unknown elements with unknown sizes.
-- Formalize handling of default element values.
-- Try to resynchronize stream when read_block fails so that failure to parse
-  a single block can be treated as non-fatal.
-- Make logging more useful to API users.
-- Avoid reparsing Cues and ignore any SeekHead at end of file.
-- Optionally build a Cue index as Clusters are parsed.
-- Support seeking without Cues.
-- Avoid building a list of Clusters as they are parsed and retain only the
-  last one parsed.
-- Add an asynchronous error code to struct nestegg and ensure that API calls
-  continue to fail safely one a fatal error has been returned.
-- Modify parser/data structures to provide a clean separation.  Perhaps the
-  parser should return a generic tree of nodes that a second pass uses to
-  initialize the main data structures.
-- Use pool allocator for all allocations.