shader_element: Make shader optional

The element is long-lived, but the shader itself isn't.

niri-visual-tests/src/cases/gradient_angle.rs

@@ -64,7 +64,7 @@ impl TestCase for GradientAngle {
         BorderRenderElement::shader(renderer)
             .map(|shader| {
                 BorderRenderElement::new(
-                    shader.clone(),
+                    shader,
                     Scale::from(1.),
                     area,
                     area,

niri-visual-tests/src/cases/gradient_area.rs

@@ -101,7 +101,7 @@ impl TestCase for GradientArea {
             BorderRenderElement::shader(renderer)
                 .map(|shader| {
                     BorderRenderElement::new(
-                        shader.clone(),
+                        shader,
                         Scale::from(1.),
                         area,
                         g_area,