render/drm_format_set: allocate using cap when duplicating
render/drm_format_set: allocate using cap when duplicating

In wlr_drm_format_dup, allocate the new wlr_drm_format using cap instead
of len. This makes it so the cap field is up-to-date and the chunk of
memory isn't too small if we append new modifiers (we don't allow this
yet but might in the future).

SirCmpwn/wlroots,swaywm/wlroots,swaywm/wlroots