Fix jump-flood outlines when using Metal backend.

Author: komadori

src/fragment.wgsl

@@ -3,8 +3,10 @@
 struct FragmentOutput {
     @location(0) colour: vec4<f32>,
 #ifdef FLAT_DEPTH
+#ifndef FLOOD_INIT
     @builtin(frag_depth) frag_depth: f32,
 #endif
+#endif
 };
 
 #ifdef ALPHA_MASK_TEXTURE
@@ -24,13 +26,15 @@ fn fragment(vertex: VertexOutput) -> FragmentOutput {
 #endif
 
 #ifdef FLAT_DEPTH
+#ifndef FLOOD_INIT
     out.frag_depth = vertex.flat_depth; 
 #endif
+#endif
 #ifdef VOLUME
     out.colour = vertex.volume_colour;
 #endif
 #ifdef FLOOD_INIT
-    out.colour = vec4<f32>(vertex.position.xy, out.frag_depth, 0.0);
+    out.colour = vec4<f32>(vertex.position.xy, vertex.flat_depth, 0.0);
 #endif
     return out;
 }