Allow to create an OpenGL Compositor with llvmpipe

Summary:
This is a radical change to the approach KWin used to have. So far KWin
fall back to XRender compositing if it detected llvmpipe. With this
change KWin will use OpenGL compositing also on llvmpipe.

This has drawbacks: it's CPU emulated which we actually do not want.
But the question is whether it still makes sense to keep the no-GL
enforcement.

There are several things to consider here:

• QtQuick will use llvmpipe anyway (including things like Plasmashell)
• glamor might be used in the xorg layer which also uses llvmpipe on XRender
• more consistent look
• we get OpenGL working in tests (important for e.g. openQA)
• hardware which does not support proper GL gets less, there are GL drivers for KVM, there are GL drivers for rPI, etc. We don't need to block it anymore because of possibly bad results on those hardware. Also they mostly use glamor.
• there is still the way to manually select XRender/QPainter compositing

Reviewers: KWin, Plasma

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D2184