### Godot version

v4.0.2.stable.official [7a0977ce2]

### System information

Windows 10, not platform specific

### Issue description

Methods `get_character_line` and `get_character_paragraph` return wrong indices for characters that are first in the respective line/paragraph. See the attached screenshot.

![image](https://user-images.githubusercontent.com/892180/236546781-692603be-eb09-419e-822e-d4152a66d9b6.png)


### Steps to reproduce

Run attached project.

The script goes through all characters in `RichTextLabel` and prints the respective character, line index, and paragraph index. None of them should be -1 since we are iterating through valid indices.

### Minimal reproduction project

[RichTextLabel.zip](https://github.com/godotengine/godot/files/11409399/RichTextLabel.zip)

int l = p_from;
	int r = p_to;
	while (l < r) {
		int m = Math::floor(double(l + r) / 2.0);
		MutexLock lock(main->lines[m].text_buf->get_mutex());
		int ofs = _calculate_line_vertical_offset(main->lines[m]);
		if (ofs < p_vofs) {
			l = m + 1;
		} else {
			r = m;
		}
	}
	return MIN(l, (int)main->lines.size() - 1);
}
_FORCE_INLINE_ float RichTextLabel::_calculate_line_vertical_offset(const RichTextLabel::Line &line) const {