Implement Turbulence properly

Or at least closer to. Now uses curl noise off of a source noise image,
documented as usable, and not an immense performance drain (just a
normal one).

Change-Id: Iac11c98cd9589cbe6a41b2b30893ab40d541d18f
Reviewed-on: http://codereview.qt-project.org/4510


Reviewed-by: Alan Alpert <alan.alpert@nokia.com>