vpxenc: Enable rate limit force keyframes events [pexhack]
vpxenc: Enable rate limit force keyframes events [pexhack]

Hard code limit to 1500 ms between each generated key frame. This
should ideally be a property and set to a value derived from RTT.

This avoids generating bursts of keyframes when receiving many
PLIs/NACKs in close succession.

1500ms is quite large, but with 500ms and lots of packetloss, the
frequent keyframe generation will more then double the bitrate from
the libvpx encoder (configured at 1mbps, producing over 2mbps),
potentially making an already bad situation much worse. By adding
another second we only see about 1,4mbps, which is more acceptable.

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