### 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)

for (int i = sr; i < fi; i++) {
			total_height = _resize_line(main, i, theme_cache.normal_font, theme_cache.normal_font_size, text_rect.get_size().width - scroll_w, total_height);
			total_height = _update_scroll_exceeds(total_height, ctrl_height, text_rect.get_size().width, i, old_scroll, text_rect.size.height);
			main->first_resized_line.store(i);
			if (stop_thread.load()) {
				return;
			}
		}
	}
	total_height = (fi == 0) ? 0 : _calculate_line_vertical_offset(main->lines[fi - 1]);
	for (int i = fi; i < (int)main->lines.size(); i++) {
		total_height = _shape_line(main, i, theme_cache.normal_font, theme_cache.normal_font_size, text_rect.get_size().width - scroll_w, total_height, &total_chars);
		total_height = _update_scroll_exceeds(total_height, ctrl_height, text_rect.get_size().width, i, old_scroll, text_rect.size.height);
		main->first_invalid_line.store(i);
		main->first_resized_line.store(i);
		main->first_invalid_font_line.store(i);
		if (stop_thread.load()) {
			return;
		}
		loaded.store(double(i) / double(main->lines.size()));
	}
	main->first_invalid_line.store(main->lines.size());
	main->first_resized_line.store(main->lines.size());
	main->first_invalid_font_line.store(main->lines.size());
	if (fit_content) {
		update_minimum_size();
	}
	emit_signal(SNAME("finished"));
}
void RichTextLabel::_invalidate_current_line(ItemFrame *p_frame) {