RichTextLabel's "fade" offsets from start of string rather than start of command

[cariad]

Tested versions

• Reproducible in v4.2.2.stable.official [15073af]

System information

Godot v4.2.2.stable - macOS 14.5.0 - GLES3 (Compatibility) - Apple M2 Max - Apple M2 Max (12 Threads)

Issue description

The documentation says the RichTextLabel's fade BBCode command counts the start offset from the command's position:

start controls the starting position of the falloff relative to where the fade command is inserted (https://docs.godotengine.org/en/4.2/tutorials/ui/bbcode_in_richtextlabel.html)

…but I believe the label actually uses the absolute position within the entire string.

Steps to reproduce

1. Create a RichTextLabel.
2. Enable BBCode and set the text to:

0123456[fade start=3 length=6]abcdefg[/fade]

I believe the label should show "0123456abc" in bright text and subsequent characters should fade from bright to dark, because I believe the "start" index of "3" should start counting from "a".

In reality, the faded "ab" implies that the "start" is being counted from the beginning of the string.

Minimal reproduction project (MRP)

N/A