Fixes#4647 Rain makes the sky black until restart. This also fixes a memory leak caused by rain adding a color layer every time step.
Reviewed-on: VoxeLibre/VoxeLibre#4648
Reviewed-by: the-real-herowl <the-real-herowl@noreply.git.minetest.land>
Co-authored-by: teknomunk <teknomunk@protonmail.com>
Co-committed-by: teknomunk <teknomunk@protonmail.com>