Usage of GL.Flush in the rendering process stalls rendering thread

[justonia]

In the Holoplay.cs script where rendering is done, there is a call to:

GL.Flush() with the comment that it helps 3D cursor ReadPixels faster.

On my machines (Pro Vega 20, RTX 2070) this introduces up to 0.15ms of GPU stalling between each render view. It can be seen in the profiler as Gfx.WaitForCommands. Given how many views get rendered each frame, this is a not-insignificant performance issue.