Formalize encodeframe.c forward delclarations

Change If4321cc5 fixed a bug caused by forward declarations not being
kept in sync across C files, resulting in a function call with the
wrong arguments. The commit moves the affected function declarations
into a header file, along with the other symbols from encodeframe.c
that were being sloppily shared.

Change-Id: I76a7b4c66d4fe175f9cbef7e52148655e4bb9ba1