UInt4 -> Float4 already existed, but not UInt -> Float.

Added the scalar conversion code and used it in the
Blitter where appropriate.

Change-Id: I9ebf63fdf8b139b960237b269f2da088f6ecac86
Reviewed-on: https://swiftshader-review.googlesource.com/10888Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>