fixed particle2d fade-out error

[zhefengzhang]

Re: https://github.com/cocos/3d-tasks/issues/19132

[github-actions]

Code Size Check Report

Wechat (WASM)	Before	After	Diff
2D Empty (legacy pipeline)	1009805 bytes	1009805 bytes	✅ 0 bytes
2D All (legacy pipeline)	2675364 bytes	2675389 bytes	⚠️ +25 bytes
2D All (new pipeline)	2767100 bytes	2767125 bytes	⚠️ +25 bytes
(2D + 3D) All	10024070 bytes	10024095 bytes	⚠️ +25 bytes

Web (WASM + ASMJS)	Before	After	Diff
(2D + 3D) All	16843132 bytes	16843157 bytes	⚠️ +25 bytes

Interface Check Report

! WARNING this pull request has changed these public interfaces:

@@ -70279,9 +70279,14 @@
         }
         export class _cocos_particle_2d_particle_simulator_2d__Particle {
             pos: math.Vec2;
             startPos: math.Vec2;
-            color: math.Color;
+            color: {
+                r: number;
+                g: number;
+                b: number;
+                a: number;
+            };
             deltaColor: {
                 r: number;
                 g: number;
                 b: number;

[zhefengzhang]

因为 Color 类型对象使用 Uint8ClampedArray 类型数组存储 rgba 数值，此类型数组的元素值添加时会对数值进行四舍五入。
而在 particle-simulator-2d.ts 中计算每帧粒子的颜色数据需要 rgba 值与 dt 浮点值相乘，最终得到的 rgba 浮点值赋值给 color 的 rgba 值时会被四舍五入，导致最终显示结果不符合预期。

Because Color type objects use an array of type Uint8ClampedArray to store rgba values, element values of this array type are rounded when added.
However, calculating the color data of each frame particle in particle-simulator-2d.ts requires multiplying the rgba value by the dt floating point value. The resulting rgba floating point value assigned to the rgba value of color will be rounded, resulting in the final display result not meeting expectations.