fix spot dir nan again (bevyengine#7176)

# Objective

fix error with shadow shader's spotlight direction calculation when direction.y ~= 0
fixes bevyengine#7152

## Solution

same as bevyengine#6167: in shadows.wgsl, clamp 1-x^2-z^2 to >= 0 so that we can safely sqrt it

crates/bevy_pbr/src/render/shadows.wgsl

@@ -49,7 +49,7 @@ fn fetch_spot_shadow(light_id: u32, frag_position: vec4<f32>, surface_normal: ve
     // construct the light view matrix
     var spot_dir = vec3<f32>((*light).light_custom_data.x, 0.0, (*light).light_custom_data.y);
     // reconstruct spot dir from x/z and y-direction flag
-    spot_dir.y = sqrt(1.0 - spot_dir.x * spot_dir.x - spot_dir.z * spot_dir.z);
+    spot_dir.y = sqrt(max(0.0, 1.0 - spot_dir.x * spot_dir.x - spot_dir.z * spot_dir.z));
     if (((*light).flags & POINT_LIGHT_FLAGS_SPOT_LIGHT_Y_NEGATIVE) != 0u) {
         spot_dir.y = -spot_dir.y;
     }