GPU acceleration in Krita

[Andrew Savonichev]

Hello,

This is another irregular update for the GPU acceleration work.

Following Dmitry's advice, I've implemented an OpenCL version of
KoCompositeOpAlphaDarken and integrated it into
KoOptimizedCompositeOpFactory.

It shows pretty good performance (up to 27x speedup) on large image
tiles, but on small tiles it is only slightly better than CPU. I'll be
looking on how to optimize this case.

Source code is published on Github[1] ('ocl' branch), and I'll be
working there until the patches are ready to go through review process.

Any feedback/suggestions are welcome.

  [1]: https://github.com/asavonic/krita-gpu

--
Andrew