Keep the state of use-opencl in one location

The real value is always the one in gegl-cl-init, instead of
being combined with the value in gegl_config().
Add fast 64bit path for RGBA u8 to gegl_memset_pattern
Convert tile-source macros to static inline functions
opencl: Make gegl_cl_color_babl() always return true

Any format can be used in an OpenCL buffer, cl_iter will already
do conversions on the CPU as needed.
Add usage help to run-compositions.py