[cli] add --core to pass arguments to the core

This adds a new commandline parameter to dt-cli, "--core". Everything
following this option will be ignored by dt-cli and just passed to the
core. Use at your own risk.

Sample usage:
darktable-cli IMG_1234.CR2 my_lovely_bw_style.xmp output.jpg --width
1200 --core -d perf -t 23
opencl: make memory mapping available
opencl: option to use pinned memory for host<->device transfer during tiling

On some devices (namely AMD) direct memory transfers may give a huge performance penalty. This
is especially visible when exporting large images when darktable needs to use tiling. Now
there is a new config parameter opencl_use_pinned_memory. Setting it to TRUE can speed
up processing by a factor of 2-3. NVIDIA devices seem to have a more efficient memory transfer
technique. They deliver better performance if opencl_use_pinned_memory is left at its default FALSE.
Merge branch 'master' into opencl