Drop direct rendering setting

The advanced compositing option "direct rendering" could only
correctly be honored in the case of proprietary NVIDIA drivers.
In all other cases playing with the setting was most likely
harmful as it could result in inconsistent states and the
option not to be honored at all.

This patch resolves this issue by moving the detection whether
to use a direct rendering context completely into the hands of
the set environment variables or the helper program:
* if LIBGL_ALWAYS_INDIRECT is set, we use an indirect context
* if KWIN_DIRECT_GL is set, we use a direct context
* if none of the two are set, we use the helper program, if it
returns 0 we create a direct context, otherwise we set
LIBGL_ALWAYS_INDIRECT and create an indirect context

If a user really wants to influence the behavior the
environment variables can be used.

REVIEW: 102074