Use -O3 by default with GCC and Clang
operations: change description of apply-lens to be the same as in GIMP
Bug 727005: "C" and "cl" gegl:pixelize produce different results

* Use input bounding box, not only its width/height, to compute
  block color for partial blocks.
* Always round down integer divisions
operations: add a cell noise operation

Added a cell noise operation implementing the texture function described in

    Steven Worley. 1996. A cellular texture basis function.
    In Proceedings of the 23rd annual conference on Computer
    graphics and interactive techniques (SIGGRAPH '96).