OpenCL implementation of pixel interpolator

This finally adds an OpenCL implementation to the pixel interpolator which is used in the demosaic module (dt_iop_clip_and_zoom). Up to now we needed to go from GPU to CPU with the full sized raw just for this interpolation step and then back to GPU with the scaled down image.

Due to intensive use of parallelization and local memory the OpenCL code should be competitive. However, as the relative speed depends a lot on GPU/CPU equipment I am interested in your benchmarking figures.