custom_ui_material border fix (#17282)

# Objective

The order of the border edges in `UiVertexOutput` is left, right, top,
bottom but in `custom_ui_material` the selectors switch them so left is
right and top is bottom.

## Solution

Reverse the conditions so that the correct border values are selected.

assets/shaders/custom_ui_material.wgsl

@@ -10,16 +10,24 @@
 
 @fragment
 fn fragment(in: UiVertexOutput) -> @location(0) vec4<f32> {
+    // normalized position relative to the center of the UI node
     let r = in.uv - 0.5;
+
+    // normalized size of the border closest to the current position
     let b = vec2(
-        select(in.border_widths.x, in.border_widths.y, r.x < 0.),
-        select(in.border_widths.z, in.border_widths.w, r.y < 0.)
+        select(in.border_widths.x, in.border_widths.y, 0. < r.x),
+        select(in.border_widths.z, in.border_widths.w, 0. < r.y)
     );
 
+    // if the distance to the edge from the current position on any axis 
+    // is less than the border width on that axis then the position is within 
+    // the border and we return the border color
     if any(0.5 - b < abs(r)) {
         return border_color;
     }
 
+    // sample the texture at this position if it's to the left of the slider value
+    // otherwise return a fully transparent color
     if in.uv.x < slider {
         let output_color = textureSample(material_color_texture, material_color_sampler, in.uv) * color;
         return output_color;