[omxplayer] Support stereo view modes with scaling

The Pi only supported a single view rectangle, which is sufficient for all mono view modes,
but only supports a subset of stereo modes.

To work around this, the scaling rectangles were ignored in 3D modes and 3D video was treated as unscaled.
While this worked or square pixel, 16:9 content on a 16:9 display, it went wrong is various other conditions.

@sraue reported that mono view of SBS/TAB content wasn't scaled correctly, and other forum reports
aspect ratio errors in widescreen 3D videos.

As it wasn't trivial to work around these bug reports, I've added a new stereo flags to the firmware
that allows a second display region to be created for the second eye, allowing scaling to work.

I've been through the video stereo modes (none, sbs, tab) and the display stereo modes (mono, sbs, tab)
and all the zoom modes, and compared the scaling to xbmc on windows and all seem to match

Requires udpated firmware.
Merge pull request #4703 from popcornmix/stereoview

[omxplayer] Support stereo view modes with scaling