Add a command line argument --gpu-vsync on/off/auto.
Prefer command line arguments for GPU settings,
as it means the value is error checked and set when Blender starts.
Also refactor GHOST_Context to take a parameter argument which
includes the stereo-visual & debug arguments and includes the newly
added vsync setting. This makes it possible to add GPU context
parameters more easily in the future.
It also removes redundant parsing & setting the VSync setting for
off-screen contexts.
Changes to recent PR !143049.
Ref !144473
5540b9440a