vpxenc: Properly handle frames with too low duration
vpxenc: Properly handle frames with too low duration

When a frame's duration is too low, calling gst_util_uint64_scale()
to scale its value can result into it being truncated to zero, which
will cause the vpx encoder to return an VPX_CODEC_INVALID_PARAM error
when trying to encode.

To prevent this from happening, we simply ignore the duration when
encoding if it becomes zero after scaling, logging a warning message.

https://bugzilla.gnome.org/show_bug.cgi?id=765391

Kurento/gst-plugins-good,Kurento/gst-plugins-good,Kurento/gst-plugins-good,Kurento/gst-plugins-good,Kurento/gst-plugins-good