Check the performance of using Utilities.Pin() instead of Interop.Fixed()

Interop.Fixed() is not available outside SharpDX, so the new Toolkit code uses Utilities.Pin(). This works, but if used in a hot path it is possible that it causes performance problems.

It would be good to check the performance difference, and see if any of the call sites are in hot paths. Also, it should be studied if we could get rid of pinning altogether.
