Renderer: Don't attempt to switch to exclusive fullscreen if "render to main" is enabled.
VideoConfig: Replace "3D Vision" by "Borderless Fullscreen" option.

This option will disable exclusive fullscreen for users who prefer the old behaviour.

Also remove the 3D Vision hack that should no longer be necessary.
CFrame: Don't reassign focus when we're not rendering to the main window.
Frame: Fix RendererHasFocus() again.
Revert "Remove fullscreen flag from video config, detect it from RendererIsFullscreen() instead."

This reverts commit cba836e6217359c4bb58ac15ef2bbf3261ee43fd.
Let the Renderer decide when to exit fullscreen.

This ensures the transition from/to exclusive mode happens while the RenderFrame is fullscreen.

This prevents fullscreen loops and relieves us of having to restore the window size after we exit fullscreen.