fix: correct coordinate rendering offset at maximum zoom via fp64 viewport center

[Copilot]

At maximum zoom levels on AMap (zoom 21+), drawn points render 20–40px away from the actual click position. The LNGLAT_OFFSET coordinate system (active when zoom > 12) uses fp64 split encoding for vertex positions, but the viewport center passed as u_ViewportCenter is a float32 uniform — silently dropping fp64LowPart(center) ≈ 6e-6°, which maps to ~18–36px error at zoom 21–22.

Changes

• scene_uniforms.glsl: Add vec2 u_ViewportCenterLow to the UBO struct (fits in existing 96-float buffer)
• projection.glsl: In the LNGLAT_OFFSET path, subtract u_ViewportCenterLow from the computed offset to achieve full double-precision center subtraction:

// Before: error = fp64LowPart(center) ≈ 6e-6° → ~20–36px at max zoom
return project_offset(vec4(X + absolutePosition64xyLow.x, Y + absolutePosition64xyLow.y, ...));

// After: vertex.x - center.x with full float64 accuracy
return project_offset(vec4(
  X + absolutePosition64xyLow.x - u_ViewportCenterLow.x,
  Y + absolutePosition64xyLow.y - u_ViewportCenterLow.y, ...
));

• CoordinateSystemService: Compute viewportCenterLow = [center[0] - Math.fround(center[0]), center[1] - Math.fround(center[1])] on every refresh()
• ICoordinateSystemService: Add ViewportCenterLow to CoordinateUniform enum and getViewportCenterLow() to the interface
• ShaderUniformPlugin: Retrieve and pack u_ViewportCenterLow into the UBO data array and WebGL1 uniforms fallback

Original prompt

---

This section details on the original issue you should resolve

<issue_title>drawPolygon绘制，光标位置和渲染的点位置偏移严重，</issue_title>
<issue_description>### 问题描述

问题描述
把高德地图放到最大时候， drawPolygon绘制，光标位置和渲染的点位置偏移严重
似乎是地图放到最大，l7坐标渲染的位置和底图坐标渲染的位置有一定误差导致的

重现链接

https://l7draw.antv.vision/example/polygon/start

重现步骤

在上面重新链接里，把地图放到最大，然后启用绘制，会发现，光标点击的位置和绘制的点位置偏移严重

预期行为

No response

平台

• 操作系统: [macOS, Windows, Linux, React Native ...]
• 网页浏览器: [Google Chrome, Safari, Firefox]

屏幕截图或视频（可选）

补充说明（可选）

No response</issue_description>

<agent_instructions>看看这个 issue，尝试解决一下</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@lzxue 应该是高精度的问题

• Fixes drawPolygon绘制，光标位置和渲染的点位置偏移严重， #2810

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