x264enc: Work around a rate control issue in libx264

x264enc: Work around a rate control issue in libx264

When variable framerate is disabled in libx264 (which occurs when using
the zerolatency tuning), libx264 ignores timestamps but still uses the
timebase leading to messed up rate control with our nanosecond timebase.

We work around this issue by setting the timebase to the reciprocal of
the framerate and we validate that the framerate is suitable.

This has been fixed upstream in libx264 but there are non-fixed versions
in the wild so this workaround is still needed.

Fixes bug #632861