Restaura l'antialiasing de fcos al port Metal de cube_lines

shaders/cube_lines/cube_lines.frag.msl

@@ -5,12 +5,12 @@ using namespace metal;
 // MSL port of cube_lines.vk.glsl. The original Shadertoy is ~780 lines and
 // uses every GLSL trick (mat3 chains, arrays, fwidth-based AA, swap macros).
 //
-// Notes on compromises in this port:
-//   - fcos() in the GLSL uses fwidth() with a fallback that depends on
-//     iResolution.y. To avoid threading iResolution through ~10 helpers, this
-//     port uses the simpler `fcos = cos` path (equivalent to defining AA_ALL
-//     in the original). Edges may be slightly less smooth than the Vulkan/GL
-//     versions; cosmetic only.
+// Notes:
+//   - fcos() uses fwidth() like the GLSL original to soften high-frequency
+//     terms. The GLSL fallback path that divides by iResolution.y when
+//     fwidth returns 0/NaN/Inf is omitted here; the main fwidth-based
+//     branch covers virtually all fragments and the fallback is rarely
+//     hit in practice.
 //   - Optional Shadertoy defines (ANIM_SHAPE, ANIM_COLOR, ROTATION_SPEED,
 //     CAMERA_*, USE_COLOR, AA_*, ONLY_BOX, etc.) are left at their defaults
 //     just like the .vk.glsl ships them.
@@ -45,7 +45,10 @@ static float3x3 rotz(float a) {
     return float3x3(float3(c, s, 0.0), float3(-s, c, 0.0), float3(0.0, 0.0, 1.0));
 }
 
-static float3 fcos(float3 x) { return cos(x); }
+static float3 fcos(float3 x) {
+    float3 w = fwidth(x);
+    return cos(x) * smoothstep(float3(3.14 * 2.0), float3(0.0), w);
+}
 
 static float3 getColor(float3 p) {
     p = abs(p);