VR / FPV mode: the picture twice, side by side, for phone headsets #204

Request from a toy-drone tester (#187/#196): "split to 2 screens, for two eyes".

What it is: the phone goes into a simple VR headset (Cardboard-style), and each eye sees one half of the screen. The app shows the same picture twice, left and right, so the pilot sees the drone's camera first person (FPV). Vendor toy apps (WIFI UAV, KY UFO) have such a VR button.

Who needs it: FPV flying of toy drones, maybe the OpenIPC FPV community. It is of little use for CCTV.

Possible steps

1. Simple: the same frame drawn twice, side by side, no lens correction. This is easy on the MJPEG/software path (toys).
2. H.264/H.265: the decoder outputs to one surface, so two copies need GL drawing, the path that caused stalls before (ANR on AG-216: RenderThread blocked in eglSwapBuffers with 8 multi-view SurfaceViews #76). Only in this mode, and with care.
3. Later: lens distortion correction and eye spacing.

Deferred until it's clear who wants it and how they would use it.