Users with disabled VSync might not expect any forced waiting on the GPU,
and the GL sync methods were intented to fix issues with VSync only,
so the new heuristic for SYNC_GL_AUTO is to enable GL sync only if
VSync is enabled, too.
Users can still request to use a specific GL sync method via the
-gl_syncmethod switch, independent of the VSync setting.
[Kp: folded `else { if () }` into `else if ()` to avoid moving
`ogl_have_ARB_sync` lines. Original change visible at
<https://github.com/dxx-rebirth/dxx-rebirth/pull/381>.]
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