chan_iax2: Prevent some needless breaking of the native IAX2 bridge.

* Clean up some twisted code in the iax2_bridge() loop.

* Add AST_CONTROL_VIDUPDATE and AST_CONTROL_SRCCHANGE to a list of frames
to prevent the native bridge loop from breaking.

* Passing the AST_CONTROL_T38_PARAMETERS frame should also allow FAX over
a native IAX2 bridge.

(issue ABE-2912)

Review: https://reviewboard.asterisk.org/r/2870/
........

Merged revisions 399697 from http://svn.asterisk.org/svn/asterisk/branches/1.8
........

Merged revisions 399708 from http://svn.asterisk.org/svn/asterisk/branches/11

For v12 and above this is really just documentation until IAX2 native
bridging is restored.
........

Merged revisions 399736 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@399737 f38db490-d61c-443f-a65b-d21fe96a405b