Creates two buffers with the size queried through the drm capabilities.
The actual cursor image is retrieved using the software cursor
functionality from the AbstractBackend and rendered into the shared
memory buffer. The the buffer is set as the new cursor image and the
rendering buffer for the cursor is swapped.
The position is updated whenever the cursor image changes or the mouse
position changes.
Uses EGL_MESA_platform_gbm to get an EglDisplay from a gbm_device.
The DrmBackend can provide a DrmBuffer for a gbm_surface and present
it.
Unfortunately buffer age seems to be slightly broken and we still have
artefacts.
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
