[vim/vim] Gvim: ligatures are reshaped when cursor moves across constituent characters (Issue #20916)

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ubaldot created an issue (vim/vim#20916)

### Steps to reproduce

1. Install Fira Code or any other font that support ligatures. Start gVim with:

- set encoding=utf-8
- set renderoptions=type:directx
- set guifont=Fira_Code:h12

2. Insert text containing a ligature sequence, for example:

`<=`

3. Move the cursor repeatedly across the two characters.

The symbol change shape:

<img width="57" height="45" alt="Image" src="https://github.com/user-attachments/assets/48dfa00c-2816-4b83-b731-ca99aef064f0" />


### Expected behaviour

The cursor keeps consistently its correct shape:

<img width="37" height="27" alt="Image" src="https://github.com/user-attachments/assets/221371cf-7556-4bca-b191-02e9fedaa263" />

Note that Vim run in Windows Terminal don't have this problem. 

### Version of Vim

9.2 with patch 1-45

### Environment

Windows 11

### Logs and stack traces

```shell

```

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<div style=3D"display: flex; flex-wrap: wrap; white-space: pre-wrap; align-=
items: center; "><img height=3D"20" width=3D"20" style=3D"border-radius:50%=
; margin-right: 4px;" decoding=3D"async" src=3D"https://avatars.githubuserc=
ontent.com/u/10845685" /><strong>ubaldot</strong> created an issue <a href=
=3D"https://github.com/vim/vim/issues/20916">(vim/vim#20916)</a></div>
<h3 dir=3D"auto">Steps to reproduce</h3>
<ol dir=3D"auto">
<li>Install Fira Code or any other font that support ligatures. Start gVim =
with:</li>
</ol>
<ul dir=3D"auto">
<li>set encoding=3Dutf-8</li>
<li>set renderoptions=3Dtype:directx</li>
<li>set guifont=3DFira_Code:h12</li>
</ul>
<ol start=3D"2" dir=3D"auto">
<li>Insert text containing a ligature sequence, for example:</li>
</ol>
<p dir=3D"auto"><code class=3D"notranslate">&lt;=3D</code></p>
<ol start=3D"3" dir=3D"auto">
<li>Move the cursor repeatedly across the two characters.</li>
</ol>
<p dir=3D"auto">The symbol change shape:</p>
<a href=3D"https://github.com/user-attachments/assets/48dfa00c-2816-4b83-b7=
31-ca99aef064f0">image.png (view on web)</a>
<h3 dir=3D"auto">Expected behaviour</h3>
<p dir=3D"auto">The cursor keeps consistently its correct shape:</p>
<a href=3D"https://github.com/user-attachments/assets/221371cf-7556-4bca-b1=
91-02e9fedaa263">image.png (view on web)</a>
<p dir=3D"auto">Note that Vim run in Windows Terminal don't have this probl=
em.</p>
<h3 dir=3D"auto">Version of Vim</h3>
<p dir=3D"auto">9.2 with patch 1-45</p>
<h3 dir=3D"auto">Environment</h3>
<p dir=3D"auto">Windows 11</p>
<h3 dir=3D"auto">Logs and stack traces</h3>
<div class=3D"highlight highlight-source-shell" dir=3D"auto"><pre class=3D"=
notranslate"></pre></div>

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