Disabling of OpenCL by default in Calc

Hey guys,

so I'd like to disable OpenCL by default in Calc. I was once again hit by
some nasty issue in the OpenCL code that makes calc unusable for me. In my
opinion we have way too many issues with it and it reduces the reliability
of Calc ( who trusts his OpenCL driver enough to do his taxes with OpenCL
enabled).

Unless someone does some huge refactoring of the OpenCL code and makes sure
that we can trust the OpenCL backend as much as our normal formula code I
would prefer if we disable it. In this case it is a regression from
40b0b9ab7703a165295b008f47df14d2ec076fb1 but that we don't detect such
fundamental problems because of the different engines and the different
devices makes it hard to rely on the OpenCL code.

I would prefer to disable the OpenCL backend by default until we have a
solution to the problems. That includes a way to detect broken drivers
automatically and a way to have a few tinderboxes that are actually
executing the OpenCL code path. Right now we are not even aware if any box
is executing the tests (and I suppose none is).

It would be nice if this could be put onto the ESC agenda so that we could
disable it in 5.2.

Regards,
Markus