fix(node): only round a box that has a border

A filled box without one is a plain rectangle: rounding it leaves the
surface showing at the corners, which reads as a seam nobody asked for.

native/test/runtime_test.cpp

@@ -145,7 +145,8 @@ int main() {
       "local root = node.create(nil, {type = 'box', w = 'fill', h = 'fill', pad = 4, gap = 2})\n"
       "local label = node.create(root, {type = 'text', label = 'Hello'})\n"
       "local button = node.create(root, {type = 'button', h = 40, interactive = true})\n"
-      "node.setStyle(root, {background = 0xFFFFFF, fill = 0xFFFFFF, color = 0, face = 0xEEEEEE, focusColor = 0xFF0000})\n"
+      "node.setStyle(root, {background = 0xFFFFFF, fill = 0xFFFFFF, color = 0, face = 0xEEEEEE,\n"
+      "  border = 0x333333, focusColor = 0xFF0000})\n"
       "assert(node.layout(root, 0, 0, 320, 240))\n"
       "local x, y, w, h = node.getRect(label)\n"
       "assert(x == 4 and y == 4 and w == 312 and h == 16)\n"
@@ -161,6 +162,8 @@ int main() {
       "node.draw(root)\n"
       "node.dropScratch()");
   assert(bench.gui.trace.find("drawText(Hello);") != std::string::npos);
+  // The bordered box rounds its corners; the button fills without one.
+  assert(bench.gui.trace.find("roundRect(fill,border);") != std::string::npos);
   assert(bench.gui.trace.find("roundRect(fill,-);") != std::string::npos);
   expectError(state, "node.create(nil, {type = 'nope'})");