Prevent DoF effect disappearing at small focus_distances (bevyengine#21816)

# Objective

- In the `depth_of_field` example, setting `focal_distance` to a small
value (<= 0.02) causes the DoF effect to disappear unexpectedly.

<img width="1936" height="1129" alt="before"
src="https://github.com/user-attachments/assets/ea12016e-b9d5-4232-b8e0-b354efdecaae"
/>

- I believe that a small focal_distance makes `(focus - f)` zero or
negative. This leads to a non-positive candidate_coc, which is then
clamped to 0.0, effectively disabling the DoF effect.

- Although the formula is physically accurate, we cannot assume users
have a deep understanding of the depth of field effect. This will make
them confused.

## Solution

- Use `max(focus - f, EPSILON)` to ensure the denominator is always
positive.

- Now, the effect looks correct.

<img width="1938" height="1135" alt="later"
src="https://github.com/user-attachments/assets/4462bc78-e5d1-4d74-a9c7-d69ed111b4c6"
/>


## Testing

- CI

---

Author: Breakdown-Dog

crates/bevy_post_process/src/dof/dof.wgsl

@@ -98,6 +98,8 @@ struct DualOutput {
 // The sampler that's used to fetch texels from the source color buffer.
 @group(1) @binding(1) var color_texture_sampler: sampler;
 
+// used to ensure `depth * (focus - f)` is always a positive number,
+const EPSILON: f32 = 1.19209290e-07;
 // cos(-30°), used for the bokeh blur.
 const COS_NEG_FRAC_PI_6: f32 = 0.8660254037844387;
 // sin(-30°), used for the bokeh blur.
@@ -128,7 +130,7 @@ fn calculate_circle_of_confusion(in_frag_coord: vec4<f32>) -> f32 {
     // This is just the formula from Wikipedia [1].
     //
     // [1]: https://en.wikipedia.org/wiki/Circle_of_confusion#Determining_a_circle_of_confusion_diameter_from_the_object_field
-    let candidate_coc = scale * abs(depth - focus) / (depth * (focus - f));
+    let candidate_coc = scale * abs(depth - focus) / (depth * max(focus - f, EPSILON));
 
     let framebuffer_size = vec2<f32>(textureDimensions(color_texture_a));
     return clamp(candidate_coc * framebuffer_size.y, 0.0, max_coc_diameter);

examples/3d/depth_of_field.rs

@@ -243,7 +243,7 @@ impl AppSettings {
 
         format!(
             "Focal distance: {:.2} m (Press Up/Down to change)
-Aperture F-stops: f/{:.2} (Press Left/Right to change)
+Aperture F-stops: f/{:.3} (Press Left/Right to change)
 Sensor height: {:.2}mm
 Focal length: {:.2}mm
 Mode: {} (Press Space to change)",