Document some of the assumptions for DMA-BUF buffer sharing and modifiers.
A wlroots user can easily get confused and think that `cap` refers to wlroots buffer capabilities, not array capacity.
This types adds a container for formats + modifiers. A list that is of [format [modifier]] was chosen instead of [format modifer] because that is how GBM accepts them. Co-Authored-By: emersion <contact@emersion.fr>
