The integration between the opencl buffer cache and GeglBuffer consists of
two (three) function calls that are conditionally done for updates are
correctly propagated between opencl / cpu.

This commit in work towards making GeglBuffer possible to reuse as a separate
libraryt without needing to link and load opencl replaces these conditionals
with a check whether a function pointer is set, and if it is set it uses it.
The opencl code sets/unsets the function pointers depending on whether it is
active.