Ivfenc will use timebase if it is set. If it is not set ivfenc will
still double the timebase so altref frames will have a unique pts.
Patch Set #3: Use integer math to generate source pts. Added a
framerate parameter. Increased the default timebase to milliseconds to
remove the *2 everywhere.

Change-Id: I8d25b5b2cb26deef7eb72d74b5f76c98cafaf4db