in a stroke of genius (total luck) I have gotten a stencil buffer which culls pixels

src/canvas.rs

@@ -486,7 +486,7 @@ impl CanvasState {
         // Specify the color to clear the framebuffer with i.e. blue
         let clear_values = vec!(
             ClearValue::Float([0.0, 0.0, 1.0, 1.0]),
-            ClearValue::DepthStencil((1.0, 1)),
+            ClearValue::DepthStencil((1.0, 0)),
         );
 
         let mut command_buffer = command_buffer.begin_render_pass(

src/canvas_shader.rs

@@ -118,13 +118,13 @@ impl CanvasShader {
              depth_write: true,
              depth_bounds_test: DepthBounds::Disabled,
              stencil_front: Stencil {
-                 compare: Compare::Always,
+                 compare: Compare::Equal,
                  pass_op: StencilOp::Invert,
                  fail_op: StencilOp::Invert,
-                 depth_fail_op: StencilOp::Invert,
+                 depth_fail_op: StencilOp::Keep,
-                 compare_mask: Some(u32::max_value()),
+                 compare_mask: Some(0x01),
-                 write_mask: Some(u32::max_value()),
+                 write_mask: Some(0xFF),
-                 reference: Some(u32::max_value()),
+                 reference: Some(0x01),
              },
              stencil_back: Default::default()
          };