Preparation for dmabuf-hints
In order to add support for the dmabuf-hints, some rework in the DRM-backend and the GL-renderer were made.
The most important changes are the introduction of struct weston_drm_format
and the fixes related to DRM_FORMAT_MOD_INVALID
(see !544 (comment 876025)).
In this MR we get these commits (which have already been reviewed and are in good shape, but more reviews are welcome!) from !544 (merged)