[platforms/drm] EGLStream DRM Backend Initial Implementation

Summary:
This is the initial implementation of a DRM backend based on the EGLDevice,
EGLOutput, and EGLStream extensions, supporting NVIDIA graphics hardware using
their proprietary driver. The new backend will be used if the environment
variable KWIN_DRM_USE_EGL_STREAMS is set. On initialization, it will attempt to
create an EGLDevice based on the DRM device currently in use and create
EGLOutputs and EGLStreams for any attached displays. These are used to control
presentation of the final composited frame. Additionally, it will register the
wl_eglstream_controller Wayland interface so that native EGL windows created by
clients can be attached to an EGLStream allowing buffer contents to be shared
with the compositor as a GL texture.

At this time there are two known bugs in the NVIDIA driver's EGL implementation
affecting desktop functionality. The first can result in tooltip windows drawn
by plasmashell to contain incorrect contents. The second prevents KWayland from
being able to query the format of EGLStream-backed buffers which interferes
with the blur effect. Fixes for both of these are currently in development and
should appear in an upcoming NVIDIA driver release.

Additionally, hardware cursors are currently not supported with this backend.
Enabling them causes the desktop to intermittently hang for several seconds.
This is also likely a bug in the NVIDIA DRM-KMS implementation but the root
cause is still under investigation.

Test Plan:
On a system with an NVIDIA graphics card running a recent release of their
proprietary driver

    * Ensure the nvidia_drm kernel module is loaded with the option "modeset=1"
      ("# cat /sys/module/nvidia_drm/parameters/modeset" should print "Y")
    * Ensure EGL external platform support is installed
      https://github.com/NVIDIA/eglexternalplatform
    * Ensure KWin was build with the CMake option
      KWIN_BUILD_EGL_STREAM_BACKEND=ON (this is the default)
    * Start a plasma wayland session with the environment variable
      KWIN_DRM_USE_EGL_STREAMS set
    * Ensure output from KWin OpenGL initialization indicates the NVIDIA EGL
      driver is in use (as opposed to Mesa / llvmpipe).
    * Desktop should be fully functional and perform smoothly.

Reviewers: #kwin, romangg, davidedmundson

Reviewed By: #kwin, romangg, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D18570