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.
There's quite some overlap and duplicated code. This AbstractEglBackend
tries to merge the two backends a little bit again.
This also introduces an AbstractEglTexture which supports both X11 and
Wayland "pixmaps" so that EglOnXBackend can support Wayland buffers.
