Changing webmenc to use libwebm for WebM file muxing.

Change-Id: I38ccc74f35c6b9684a87d04d8f9354dbe9a3263b

Temporary revert.
Problems with conflicting definitions of type off_t
in MSVC builds that need resolving.

c:\Program Files (x86)\
  Microsoft Visual Studio 9.0\VC\include\wchar.h(479) :
  "error C2371: 'off_t' : redefinition; different basic types
c:\on2experimental\libvpx\tools_common.h(26) :
   see declaration of 'off_t'"

This reverts commit 92a4c591.

Change-Id: I535e40a18842a92e3e6e0b29e5fba66313010803

Changing webmenc to use libwebm for WebM file muxing.

Change-Id: I42eb688953865003214c05bdf2076ee00db28970

Change-Id: I5164df72aff84eca0ace56032c5373f04053c6a5

I'm sure I could do more, but I don't know how long this code has to
live. I think this at least makes the code a little easier to read and
understand.

Change-Id: I6ca76357f89468d4851a3d1826e7aefa498e51d1

This is mainly a clean up patchset. It moves the WebM writing support
out of vpxenc and into its own source file. Changes to tools_common and
vpxdec result from relocation of shared bits of code.

Change-Id: Iee55d3285f56e0a548f791094fb14c5ac5346a26