Errors with build checks on Mac OS (check_mac_release())

[cornelmpop]

A test in test-mesh_mark_pois.R (expect_known_scene("mesh_mark_pois_demosphere_vp1", close = TRUE)) fails on the Mac OS builder (https://mac.r-project.org/macbuilder/submit.html), apparently due to a trivial and inconsequential change in material color for a known scene. Some warnings are also generated.

[cornelmpop]

All checks pass (and only a NOTE is generated) with the expect_known_scene calls commented out. Note that the first call (line 48 of test-mesh_mark_pois.R in version 0.5.2) does seem to work fine.