### Godot version

v4.2.rc1.official [ad72de508]

### System information

Godot v4.2.rc1 - Windows 10.0.22621 - Vulkan (Forward+) - dedicated NVIDIA GeForce GTX 1660 Ti (NVIDIA; 31.0.15.3742) - Intel(R) Core(TM) i7-10700F CPU @ 2.90GHz (16 Threads)

### Issue description

While trying to use a custom effect script as a workaround for the [rainbow bbcode effect](https://github.com/godotengine/godot/issues/85031) at the RichTextLabel I found a bug regarding the `CharFXTransform.relative_index`.
The variable is explained as follows in the [documentation](https://docs.godotengine.org/en/4.0/classes/class_charfxtransform.html#class-charfxtransform-property-relative-index): 
```
The character offset of the glyph, relative to the current RichTextEffect custom block.
```
However, when using the script below, the character index shifts one to the left each time instead of being at the same position for each new effect block.
![grafik](https://github.com/godotengine/godot/assets/71942164/7b1b0557-d515-4d56-a25e-d2a2810b2966)


### Steps to reproduce

Add this script as Custom Effect to the RichTextLabel (or see the MRP):
```@tool
extends RichTextEffect
class_name test

var bbcode = "test"

func _process_custom_fx(char_fx):
	if char_fx.relative_index == 5:
		char_fx.color = Color.BLACK
	pass
```
Add this Text to a Rich TextLabel:
```
[test]0123456789[/test]
[test]0123456789[/test]
[test]0123456789[/test]
[test]0123456789[/test]
[test]0123456789[/test]
[test]0123456789[/test]
``` 

### Minimal reproduction project

[relative_index_issue.zip](https://github.com/godotengine/godot/files/13462368/relative_index_issue.zip)

if (!is_visible()) {
		return 0;
	}
	const_cast<RichTextLabel *>(this)->_validate_line_caches();
	return visible_paragraph_count;
}
void RichTextLabel::scroll_to_line(int p_line) {
	if (p_line <= 0) {
		vscroll->set_value(0);
		return;
	}
	_validate_line_caches();
	int line_count = 0;
	int to_line = main->first_invalid_line.load();
	for (int i = 0; i < to_line; i++) {
		MutexLock lock(main->lines[i].text_buf->get_mutex());
		if ((line_count <= p_line) && (line_count + main->lines[i].text_buf->get_line_count() >= p_line)) {
			float line_offset = 0.f;
			for (int j = 0; j < p_line - line_count; j++) {
				line_offset += main->lines[i].text_buf->get_line_size(j).y + theme_cache.line_separation;
			}
			vscroll->set_value(main->lines[i].offset.y + line_offset);
			return;
		}
		line_count += main->lines[i].text_buf->get_line_count();
	}
	vscroll->set_value(vscroll->get_max());
}
float RichTextLabel::get_line_offset(int p_line) {