### 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)

TS->font_draw_glyph_outline(frid, ci, glyphs[i].font_size, p_shadow_outline_size, fx_offset + char_off + p_shadow_ofs, gl, modulated_shadow_color);
							}
						}
						if (modulated_outline_color.a != 0.0 && size > 0) {
							Transform2D char_reverse_xform;
							char_reverse_xform.set_origin(-char_off);
							Transform2D char_final_xform = char_xform * char_reverse_xform;
							draw_set_transform_matrix(char_final_xform);
							TS->font_draw_glyph_outline(frid, ci, glyphs[i].font_size, size, fx_offset + char_off, gl, modulated_outline_color);
						}
					}
					processed_glyphs_ol++;
				}
				gloff.x += glyphs[i].advance;
			}
		}
		draw_set_transform_matrix(Transform2D());
		Vector2 fbg_line_off = off + p_ofs;
		
		Vector2i chr_range = TS->shaped_text_get_range(rid);
		_draw_fbg_boxes(ci, rid, fbg_line_off, it_from, it_to, chr_range.x, chr_range.y, 0);
		
		Color selection_bg = theme_cache.selection_color;
		int sel_start = -1;
		int sel_end = -1;
		if (selection.active && (selection.from_frame->lines[selection.from_line].char_offset + selection.from_char) <= (l.char_offset + TS->shaped_text_get_range(rid).y) && (selection.to_frame->lines[selection.to_line].char_offset + selection.to_char) >= (l.char_offset + TS->shaped_text_get_range(rid).x)) {
			sel_start = MAX(TS->shaped_text_get_range(rid).x, (selection.from_frame->lines[selection.from_line].char_offset + selection.from_char) - l.char_offset);
			sel_end = MIN(TS->shaped_text_get_range(rid).y, (selection.to_frame->lines[selection.to_line].char_offset + selection.to_char) - l.char_offset);
			Vector<Vector2> sel = TS->shaped_text_get_selection(rid, sel_start, sel_end);
			for (int i = 0; i < sel.size(); i++) {