Limit size of initial keyframe in one-pass.

Rather than using a default size of 1/2 or 3/2 seconds for the first
frame, use a fraction of the initial buffer level to give the
application some control.

This will likely undergo further refinement as size limits on key
frames are currently under discussion on codec-devel@, but this gives
much better behavior for small buffer sizes as a starting point.

Change-Id: Ieba55b86517b81e51e6f0a9fe27aabba295acab0