Summary:
The egl implementation for the virtual platform tries to use a render
node if available. If there is no render node it looks for a virtual
(kernel driver vgem) device, which unfortunately does not create a
render node in mainline kernel (there are patches in ChromiumOS).
For this the Udev wrapper is extended to search for renderNode devices
and for virtual dri devices.
If either render node or vgem dri device is found, it is tried to be
opened (without logind escalation) and on success a gbm device is
created on it. If any step of this fails the so far default behavior
of default device is tried for creating the EGLDisplay.
All of this is compile optional, so that the virtual platform does not
hard depend on udev and/or gbm.
Test Plan:
Auto tests which need OpenGL executed and verified that they
use the render node or vgem device.
Reviewers: #kwin, #plasma_on_wayland
Subscribers: plasma-devel, kwin
Tags: #plasma_on_wayland, #kwin
Differential Revision: https://phabricator.kde.org/D2216
Introduces a new (optional) dependency: libdrm.
The DrmBackend currently supports finding the first connected output.
It can create shared memory buffers which are used by SceneQPainter to
do double buffered rendering.
There is still lots to do, the following things are not yet working:
* multiple outputs
* page flip
* OpenGL (through gbm)
* restoring mode setting to start value
