Fix multiple SVG import rendering bugs in masks and gradient strokes.

[OnionsYu]

本次修复了 SVG 导入过程中三处影响渲染正确性的问题：

1. mask-type 属性未生效：之前固定使用 Luminance 计算遮罩，忽略了 SVG 标准的 mask-type 属性。现在会读取 mask-type，"alpha" 时使用 MaskType::Alpha，其余情况保持 Luminance 默认行为。

2. 渐变描边 stroke-opacity 失效：原实现仅在纯色分支处理 stroke-opacity，导致 url() 引用的渐变描边丢失透明度。已将 stroke-opacity 解析提前到分支之前，使其同时作用于纯色和渐变描边。

3. 隐藏 mask 引用图层与可见图层被合并：isSimpleShapeLayer 在合并简单形状图层时未区分可见性，会把不可见的 mask 引用层与同形状的可见图层合并，导致可见图层的 painter 丢失、画面消失。现在显式拒绝合并不可见图层。

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.55%. Comparing base (9e5ef77) to head (1e33b13).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3503      +/-   ##
==========================================
- Coverage   80.57%   80.55%   -0.03%     
==========================================
  Files         620      620              
  Lines       67021    67024       +3     
  Branches    19885    19887       +2     
==========================================
- Hits        54002    53988      -14     
- Misses       9083     9086       +3     
- Partials     3936     3950      +14     

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