Render faster if possible on fixed time step

Currently when MonoGame is using a fixed time step, it won't ever render faster than 60FPS, despite the GPUs capability to do so.

This modifies the tick so that if SynchronizeWithVerticalRetrace is off, it will continue to render frames until the time is used up (instead of sleeping).

This is useful for diagnosing whether rendering is responsible for causing slow frames by giving an accurate representation of how fast the current frame can be rendered.