### 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)

}
					yofs += frame->padding.size.y;
					offset.x += table->columns[column].width + theme_cache.table_h_separation + frame->padding.size.x;
					row_height = MAX(yofs, row_height);
					if (column == col_count - 1) {
						offset.x = 0;
						row_height += theme_cache.table_v_separation;
						table->total_height += row_height;
						offset.y += row_height;
						table->rows.push_back(row_height);
						table->rows_baseline.push_back(table->total_height - row_height + row_baseline);
						row_height = 0;
					}
					idx++;
				}
				int row_idx = (table->align_to_row < 0) ? table->rows_baseline.size() - 1 : table->align_to_row;
				if (table->rows_baseline.size() != 0 && row_idx < (int)table->rows_baseline.size() - 1) {
					l.text_buf->resize_object((uint64_t)it, Size2(table->total_width, table->total_height), table->inline_align, Math::round(table->rows_baseline[row_idx]));
				} else {
					l.text_buf->resize_object((uint64_t)it, Size2(table->total_width, table->total_height), table->inline_align);
				}
			} break;
			default:
				break;
		}
	}
	l.offset.y = p_h;
	return _calculate_line_vertical_offset(l);
}
float RichTextLabel::_shape_line(ItemFrame *p_frame, int p_line, const Ref<Font> &p_base_font, int p_base_font_size, int p_width, float p_h, int *r_char_offset) {