When display layout is changed to overlap outputs the background is incorrectly updated

[AlanGriffiths]

Dynamically updating the display layout become possible with Miral 3.8 and #122 (or #126), so this only affects edge.

When switching from side_by_side to default (with the following config) the background from the smaller external display is visible over background on the larger display.

display configuration

layouts:
# keys here are layout labels (used for atomically switching between them).
# The yaml anchor 'the_default' is used to alias the 'default' label

  default:
    cards:
    # a list of cards (currently matched by card-id)

    - card-id: 0
      eDP-1:
        # This output supports the following modes: [email protected]
        #
        # Uncomment the following to enforce the selected configuration.
        # Or amend as desired.
        #
        state: enabled	# {enabled, disabled}, defaults to enabled
        mode: [email protected]	# Defaults to preferred mode
        position: [0, 0]	# Defaults to [0, 0]
        orientation: normal	# {normal, left, right, inverted}, defaults to normal
        scale: 1
        group: 0	# Outputs with the same non-zero value are treated as a single display

      DisplayPort-1:
        # (disconnected)

      HDMI-A-1:
        # (disconnected)

      DisplayPort-2:
        # (disconnected)

      HDMI-A-2:
        # This output supports the following modes: [email protected], [email protected],
        # [email protected], [email protected], [email protected], [email protected]
        #
        # Uncomment the following to enforce the selected configuration.
        # Or amend as desired.
        #
        state: enabled	# {enabled, disabled}, defaults to enabled
        mode: [email protected]	# Defaults to preferred mode
        position: [0, 0]	# Defaults to [0, 0]
        orientation: normal	# {normal, left, right, inverted}, defaults to normal
        scale: 1
        group: 0	# Outputs with the same non-zero value are treated as a single display

  side_by_side:
    cards:
    # a list of cards (currently matched by card-id)

    - card-id: 0
      eDP-1:
        # This output supports the following modes: [email protected]
        #
        # Uncomment the following to enforce the selected configuration.
        # Or amend as desired.
        #
        state: enabled	# {enabled, disabled}, defaults to enabled
        mode: [email protected]	# Defaults to preferred mode
        position: [0, 0]	# Defaults to [0, 0]
        orientation: normal	# {normal, left, right, inverted}, defaults to normal
        scale: 1
        group: 0	# Outputs with the same non-zero value are treated as a single display

      DisplayPort-1:
        # (disconnected)

      HDMI-A-1:
        # (disconnected)

      DisplayPort-2:
        # (disconnected)

      HDMI-A-2:
        # This output supports the following modes: [email protected], [email protected],
        # [email protected], [email protected], [email protected], [email protected]
        #
        # Uncomment the following to enforce the selected configuration.
        # Or amend as desired.
        #
        state: enabled	# {enabled, disabled}, defaults to enabled
        mode: [email protected]	# Defaults to preferred mode
        position: [2560, 0]	# Defaults to [0, 0]
        orientation: normal	# {normal, left, right, inverted}, defaults to normal
        scale: 1
        group: 0	# Outputs with the same non-zero value are treated as a single display