videorate: support frame-by-frame for multiview-mode

Submitted by Hyunjun Ko @zzoon

Link to original bug (#784372)

Description

Recently I've worked for MVC encoding using vaapi.
Basically it's been successful with static pipeline, but I found one issue when I test with encodebin.

My test pipeline is as the following:

gst-launch-1.0 gltestsrc ! glviewconvert input-mode-override=mono output-mode-override=frame-by-frame ! glcolorconvert ! gldownload ! encodebin profile="video/mpegts, systemstream=true, packetsize=188:video/x-h264" ! filesink location=mvc_va.ts

I thought it's working fine but I realized it's not.
That is videorate drops one of two frames every time by comparing timestamp of each buffer, which is what videorate should be doing normally.

So I'm asking this question.
Do we need to implement something in videorate to support multiview-mode, especially for frame-by-frame?
Or is there another solution for doing this?