This commit makes it clear that compositors can send valid modifiers and
DRM_FORMAT_MOD_INVALID for a given format. This means that the compositor
supports both implicit and explicit modifiers. See the warning further
down:

> Warning: It should be an error if the format/modifier pair was not
> advertised with the modifier event. This is not enforced yet because
> some implementations always accept DRM_FORMAT_MOD_INVALID. Also
> version 2 of this protocol does not have the modifier event.

Xwayland already requires compositors to send DRM_FORMAT_MOD_INVALID
for importing buffers with an implicit modifier [1].

In a future protocol version, it would be nice to make it a protocol
error (or at least a soft failure) to use any format/modifier pair that
wasn't advertised. A use-case for this is Vulkan compositors: the Vulkan
DMA-BUF extensions require an explicit modifier and cannot import
buffers which have an implicit modifier.

[1]: https://gitla...