Fullscreen (native): use current video mode, not highest of monitor
Fullscreen (native): use current video mode, not highest of monitor

This prevents unnecessary transitions between full/windowed, and makes
it smooth, also fixes unintentional aspect ratio changes -- such as if
you have an Oculus DK2 connected in mirrored mode, its native resolution
is 1920x1080@75Hz, but if you pick the highest 1600x2560, then it will
be shown with black bars on the side as a tall, long strip.

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