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color: deliver crucial, not all information

Pekka Paalanen requested to merge pq/wayland-protocols:mr/details into color

This MR has two patches. The first one is quoted below even though it is a two-liner, and the second one addresses a review comment in a simple way.

There could be two kinds of additions to wp_image_description_v1 events:

  • Crucial ones, without which the description is useless, for example some completely new form of information that doesn't translate to old events. E.g. a standard appears for a profile file format that is utterly incompatible with ICC v4 files.

  • Backwards compatible ones, which are additional details to existing forms of information. E.g. viewing environment parameters. In case these details are not possible to deliver, we know what assumptions are made, and can account for those.

When crucial events are missing, there is nothing one can do. In reality the difference is not that obvious.

When additional detail events are missing, and it actually is possible to have reasonable assumptions that hold, a compositor could deliver what it can and follow the assumptions clients have done. However, this means that the preferred or output image description is not what the compositor actually has, it is something different, so the compositor must take that into account. Therefore the backward compatible case should be used only when it is necessary to keep existing applications working as well as they did before.

This is to avoid the case where one adds a minor detail in the events, and suddenly all color critical applications that do their own color management stop being able to do their job because the compositor refuses to communicate anything.

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