Changing how FShaders work a bit

See http://www.reddit.com/r/futile/comments/1t9jtn/blurshader/

I also added a blurshader (by jpsarda) temporarily.
Changes to FSliceButton
RXPerfTester random bool
Node GetPositionRelativeToAncestor

Kind of a simpler version of LocalToOther, but with some interesting
properties.
FContainer.AddChildAtBack

Syntactic sugar for AddChildAtIndex(0)