Fix gizmo line width issue when using perspective (#9067)

# Objective

- In bevy_polyline, we discovered an issue that happens when line width
is smaller than 1.0 and using perspective. It would sometimes end up
negative or NaN. I'm not entirely sure _why_ it happens.

## Solution

- Make sure the width doesn't go below 0 before multiplying it with the
alpha

# Notes 

Here's a link to the bevy_polyline issue
https://github.com/ForesightMiningSoftwareCorporation/bevy_polyline/issues/46

I'm not sure if the solution is correct but it solved the issue in my
testing.

Co-authored-by: François <mockersf@gmail.com>

crates/bevy_gizmos/src/lines.wgsl

@@ -64,7 +64,7 @@ fn vertex(vertex: VertexInput) -> VertexOutput {
 #endif
 
     // Line thinness fade from https://acegikmo.com/shapes/docs/#anti-aliasing
-    if line_width < 1. {
+    if line_width > 0.0 && line_width < 1. {
         color.a *= line_width;
         line_width = 1.;
     }