Relocate it to tools_common.h so we can stop duping this code
everywhere.

BUG=https://code.google.com/p/webm/issues/detail?id=660

Change-Id: If8aa49b74d297273cd9fd7006b2767837055a359

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