MGFX Reflection Fix

This is a cleaned up version of PR #1846 by @nightst4r which addresses issue #1845.

It fixes cases where we were marshaling within a union which can cause type reflection to fail.  This then causes our content processors to fail to be loaded by XNA on some systems.  Removing the marshaling within unions fixes this.

Note this does not affect effect processing at all really.  The unions in question are not being used by MGFX.