Discuss correct way to get per-plane FD using mapper metadata API.

Approach 0:
(Used by mesa3d EGL importer for as generic solution for some time)
Use buffer_handle_t->data[plane_idx or 0] as FD.

Approach 1:

One approach is having each vendor define their custom metadata to access the fds. For drm_hwcomposer this would mean having to support all these different ways of retrieving the fds. In this case, we would indeed not have a custom AIDL in drm_hwcomposer, but would have to support the ones each customer provides.

Approach 2:

To have drm_hwcomposer define its custom metadata. Interested vendors can then add support for drm_hwcomposer in their Gralloc implementations.