Add user mendel to group render.

The render group is created by systemd and membership is required to
use DRM render nodes.

This was changed in buster:
https://salsa.debian.org/systemd-team/systemd/commit/e3772a013721083a740ab9dedbf060cf5b3c3709

BUG: 144855296
Change-Id: I5976afbadc5d80f4ff7d602dc8874b82a5f69f65