For each DrmOutput a gbm_surface and EglSurface is created. When
rendering per screen the context is made current on each of the
surfaces.
Note: viewport handling needs to be improved by e.g. passing through
the scene to restore to correct viewport after dropping an FBO.
Furthermore it seems like buffer age is not working correctly in this
setup (not overly surprising).
Dynamic changes are not yet supported.
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.
