Cycles: Add a UI warning, in case the experimental GPU kernel is used.
Cycles: Add a UI warning, in case the experimental GPU kernel is used.

The experimental kernel is slower and can cause issues on some cards still, so better communicate it well.

tangent-opensource/coreBlackbird,pyrochlore/cycles,pyrochlore/cycles,tangent-opensource/coreBlackbird,tangent-opensource/coreBlackbird,pyrochlore/cycles