shaders: Optimize rounding_alpha() a little

Simplified clamping exploiting half_px + half_px = 1.0 / niri_scale.

Makes the resize shader fit on a Eee PC ALU.

src/render_helpers/shaders/rounding_alpha.frag

@@ -19,6 +19,9 @@ float niri_rounding_alpha(vec2 coords, vec2 size, vec4 corner_radius) {
     }
 
     float dist = distance(coords, center);
-    float half_px = 0.5 / niri_scale;
-    return 1.0 - smoothstep(radius - half_px, radius + half_px, dist);
+
+    // Manual smoothstep() between radius - half_px and radius + half_px
+    // to avoid a division in clamp().
+    float t = clamp((dist - radius) * niri_scale + 0.5, 0.0, 1.0);
+    return 1.0 - t * t * (3.0 - 2.0 * t);
 }