Partially overlapping outputs misrendered when scale ≠ 1

[RAOF]

Affects (at least) gbm-kms and atomic-kms.

OverlappingOutputGroup calculates in logical coördinates, but at the hardware level outputs need to be using physical coördinates. It doesn't group outputs with dissimilar scales together, but if both outputs have the same scale and are partially overlapping the scanout position of the one that's not top-left will be incorrect.

You can see this behaviour with the following configuration taken from my laptop; the active ingredient is “position: [1919, 0]”. You can see that changing back between 1919 (so the outputs are grouped into a single overlapping group which should share only a single line of pixels) and 1920 (so that the outputs are distinct) results in drastic output changes.

layouts:
  default:                         # the current layout
    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: 2
        group: 0        # Outputs with the same non-zero value are treated as a single display

      DisplayPort-1:
        # (disconnected)

      DisplayPort-2:
        # (disconnected)

      DisplayPort-3:
        # This output supports the following modes: [email protected], [email protected],
        # [email protected], [email protected], [email protected], [email protected], [email protected],
        # [email protected], [email protected], [email protected], [email protected], [email protected],
        # [email protected], [email protected], [email protected], [email protected], [email protected],
        # [email protected], [email protected], [email protected], [email protected], [email protected]
        # [email protected], [email protected], [email protected], [email protected], [email protected],
        # [email protected], [email protected], [email protected], [email protected], [email protected],
        # [email protected], [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: [1919, 0]        # Defaults to [0, 0]
        orientation: normal     # {normal, left, right, inverted}, defaults to normal
        scale: 2
        group: 0        # Outputs with the same non-zero value are treated as a single display

[RAOF]

(This no longer affects the atomic-kms platform)