iOS 8 beta 4 support (Metal changes)

I have checked in a change to support beta 4 (specifically Metal).

The change can be found here:

I didn’t force a recompile of all shaders, so if you have any problems with shaders acting funny, try changing a digit in the GUID in Engine/Shaders/ShaderVersion.usf and cook again.

Also, make sure to get this fix if you are seeing a full gray screen (which is a workaround to a bug in iOS 8 affecting GL and Metal):

Lemme know if you have any issues! I am subscribed to this thread.