fix: 'box' primitive uv & update stairs model

src/maths/UVRegion.hpp

@@ -43,8 +43,8 @@ struct UVRegion {
     }
 
     void scale(float x, float y) {
-        float w = getWidth();
-        float h = getHeight();
+        float w = u2 - u1;
+        float h = v2 - v1;
         float cx = (u1 + u2) * 0.5f;
         float cy = (v1 + v2) * 0.5f;
         u1 = cx - w * 0.5f * x;
@@ -63,4 +63,10 @@ struct UVRegion {
         u2 = vec.z;
         v2 = vec.w;
     }
+
+    UVRegion operator*(const glm::vec2& scale) const {
+        auto copy = UVRegion(*this);
+        copy.scale(scale);
+        return copy;
+    }
 };