Draft: Prevent threading when OpenCL is enabled

This commit resolves an issue that arose when a GEGL graph operation utilizing threading was combined with an OpenCL operation, resulting in artifacts within the final image.

[*] gegl/gegl-parallel.c

• Incorporates a check for OpenCL usage to determine whether threading should be disabled.

[*] gegl/gegl-parallel.h

• Documents the behavior of disabling threading when OpenCL is enabled.

Tested with all tests. All tests passed successfully except for the already failing ones (cross verified with the master branch) on the stock test image.

This commit has an indirect consequence, fixes #252. For focus-blur, the issue lies with the implementation piecewise-blend which is used by variable blur utilizing multithreading when OpenCL is enabled. This commit addresses the issue in general and applies to all issues from GEGL's graph consisting of a mix of threaded and OpenCL implementations.