Slate resource preloading

Hi Jamie,

I confirm that this work very well. No flickering is apparent anymore.

Thanks a lot !

This change is now available in the git master, should you want to cherry pick it rather than apply the fix by hand. The fix itself will be in the 4.2 release.

https://github.com/EpicGames/UnrealEngine/commit/28e0d833f51958df8d8436ac240aa6633c3a8c9d