Summary:
The current way to specify the OpenGL context attributes does no longer
scale as can be seen in D6344. There are too many different context
attribute sets and with every addition we grow lots of copied code. The
chances to introduce errors in that code which is difficult to debug are
very high. As can be seen in the glx backend which defines major 1,
minor 2, but it should be major 2, minor 1.

This change reworks this code by creating a builder class which contains
only an abstract definition of what needs to be in the attributes.
E.g. the version, whether it's robust and so on.

Now we can just have a list of possible attributes in a well described
way:
    auto builder;
    builder.setVersion(3, 1);
    builder.setRobust(true);

All possible builders are added to a list and operated on in a for loop
which tries to creat a context. Once it succeeded it breaks the list.
In addition a debug statement is added which prints out the set of
options which went into the context.

So far this is only done for EGL, GLX can follow once D6344 is merged.

Test Plan: New unit test added, kwin_wayland OpenGL tests run and verified

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D6396