### 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 (font_it->font_size > 0) {
					font_size = font_it->font_size;
				}
			}
			ItemFontSize *font_size_it = _find_font_size(it);
			if (font_size_it && font_size_it->font_size > 0) {
				font_size = font_size_it->font_size;
			}
			TS->shaped_set_span_update_font(t, i, font->get_rids(), font_size, font->get_opentype_features());
		}
	}
	Item *it_to = (p_line + 1 < (int)p_frame->lines.size()) ? p_frame->lines[p_line + 1].from : nullptr;
	for (Item *it = l.from; it && it != it_to; it = _get_next_item(it)) {
		switch (it->type) {
			case ITEM_TABLE: {
				ItemTable *table = static_cast<ItemTable *>(it);
				for (Item *E : table->subitems) {
					ERR_CONTINUE(E->type != ITEM_FRAME); 
					ItemFrame *frame = static_cast<ItemFrame *>(E);
					for (int i = 0; i < (int)frame->lines.size(); i++) {
						_update_line_font(frame, i, p_base_font, p_base_font_size);
					}
				}
			} break;
			default:
				break;
		}
	}
}
float RichTextLabel::_resize_line(ItemFrame *p_frame, int p_line, const Ref<Font> &p_base_font, int p_base_font_size, int p_width, float p_h) {