libgui|GLFramebuffer: Stereo left/right swap mode

Even though rendering always happens in Canvas’s GL frame buffer,
VR mode 13 should now theoretically work as GLFramebuffer can be
told to use a different swap mode for the left/right frames.

In practice, the “left” swap mode draws the contents of the Canvas
frame buffer into the window’s BACK_LEFT buffer; same goes for the
“right” swap mode. The “stereo” swap will then simply do a regular
back-to-front swap with the finished contents.
Renderer: Use the left/right stereo swap mode in VR mode 13 (quad-buffered)

Todo: Test this on actual hardware.

IssueID #1680