Corrupted graphics on AMD Radeon Pro

[iceiix]

Steven on a system with an AMD Radeon Pro 580 8192 MB video card (on macOS 10.14 Mojave, but doesn't occur on a different machine with the same OS but an Intel card) is able to build, run and connect to a server, but the video is severely mangled:

You can almost make out the world, and it changes as you move (no mouse bug like on Linux https://github.com/Thinkofname/steven/issues/73 on mac since it doesn't use Wayland), but is not entirely usable. Commenting out most of the rendering except the sky reveals this striped pattern:

in full screen it is completely striped, each band is 16 pixels:

where the light blue is the sky color, which is expected to cover the full screen. The dark blue is solid 0,0,255.

The problem appears to be related to sampling. If I make this change:

diff --git a/src/render/mod.rs b/src/render/mod.rs
index 9644d5d..1eb9b5e 100644
--- a/src/render/mod.rs
+++ b/src/render/mod.rs
@@ -42,7 +42,7 @@ use std::sync::mpsc;
 const ATLAS_SIZE: usize = 1024;
 
 // TEMP
-const NUM_SAMPLES: i32 = 1;
+const NUM_SAMPLES: i32 = 2;
 
 pub struct Camera {
     pub pos: cgmath::Point3<f64>,

then it renders correctly! But I don't completely understand why, or if there is a better fix. Any ideas.