Fix gizmo lines deforming or disappearing when partially behind the camera (#9470)

If a line has one point behind the camera(near plane) then it would
deform or, if the `depth_bias` setting was set to a negative value,
disappear.

## Solution

The issue is that performing a perspective divide does not work
correctly for points behind the near plane and a perspective divide is
used inside the shader to define the line width in screen space.
The solution is to perform near plane clipping manually inside the
shader before the perspective divide is done.

Author: tim-blackbird

crates/bevy_gizmos/src/lines.wgsl

@@ -43,8 +43,13 @@ fn vertex(vertex: VertexInput) -> VertexOutput {
     let position = positions[vertex.index];
 
     // algorithm based on https://wwwtyro.net/2019/11/18/instanced-lines.html
-    let clip_a = view.view_proj * vec4(vertex.position_a, 1.);
-    let clip_b = view.view_proj * vec4(vertex.position_b, 1.);
+    var clip_a = view.view_proj * vec4(vertex.position_a, 1.);
+    var clip_b = view.view_proj * vec4(vertex.position_b, 1.);
+
+    // Manual near plane clipping to avoid errors when doing the perspective divide inside this shader.
+    clip_a = clip_near_plane(clip_a, clip_b);
+    clip_b = clip_near_plane(clip_b, clip_a);
+
     let clip = mix(clip_a, clip_b, position.z);
 
     let resolution = view.viewport.zw;
@@ -92,6 +97,18 @@ fn vertex(vertex: VertexInput) -> VertexOutput {
     return VertexOutput(clip_position, color);
 }
 
+fn clip_near_plane(a: vec4<f32>, b: vec4<f32>) -> vec4<f32> {
+    // Move a if a is behind the near plane and b is in front. 
+    if a.z > a.w && b.z <= b.w {
+        // Interpolate a towards b until it's at the near plane.
+        let distance_a = a.z - a.w;
+        let distance_b = b.z - b.w;
+        let t = distance_a / (distance_a - distance_b);
+        return a + (b - a) * t;
+    }
+    return a;
+}
+
 struct FragmentInput {
     @location(0) color: vec4<f32>,
 };