When the vaapi decoder is used in copy mode, it creates a dummy
display to render to. In theory, this should support hardware
decoding on on a separate GPU that is not actually connected to
any output (like an iGPU which supports more formats than the
external GPU to which the monitor is connected).
However, before this change, only X11 displays were supported as
dummy displays. This caused some graphics drivers (namely
intel-driver) to core dump when they were not actually used as X11
module.
This change introduces support for drm libav displays, which
allows vaapi-copy to run on such cards which are not actually
rendering the X11 output.
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