Starting with commit 4cde507be6a116 "backend-drm: fix plane sorting" the
plane list will have a descending order of the planes rather than ascending.
This reversed order had the side-effect of exposing the fact that we
don't set-up a plane index when creating the drm_plane using the DRM
virtual API. Without settting a plane index for that drm_plane we
effectively overwrite the plane index which has the 0 (zero) entry.
This wasn't an issue before commit 4cde507be6a116 "backend-drm: fix
plane sorting" as it seems we never picked up that plane index as
being a suitable one due to the fact that those were assigned to primary
planes, but after that commit, the cursor plane will be one getting
the 0 (zero) plane index.
Finally, this would trip over because we attempt to place a (cursor)
view on a primary plane (where it would've normally be a cursor
plane) and we end up with no framebuffer ref.
This is fixed trivially by assigning a plane index, different than the
ones already created by create_spirtes().
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
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