To support heterogeneous outputs, the output must be created by the
same backend as the head(s) it is created for. Solve this by always
creating an output with a first head to attach that determines the
backend to use. Skip already attached first heads in drm_try_attach().
See: https://gitlab.freedesktop.org/wayland/weston/-/issues/268
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
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