fix(backlight): %backlight% token appearing on the bar (#3081)

* Fix backlight initial value matching the actual brightness level

The initial value is `0`. If the actual brightness is also `0` you see
`%percentage%` token as text, it’s not being replaced. Since
`m_brightness` is an `int` the initial value could be just `-1` since
the actual brightness is never goes below zero. Thus this bug will never
show up again because actual brightness on first render won’t match
the “old” `m_brightness` value.

See also for more detailed explanation of the bug:
https://github.com/polybar/polybar/discussions/3079#discussioncomment-8169932

* Add docs

---------

Co-authored-by: Viacheslav Lotsmanov <lotsmanov89@gmail.com>

CHANGELOG.md

@@ -9,6 +9,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
+### Fixed
+- `internal/backlight`: Module could display the literal `%percentage%` token if the backlight reports a value of 0 at startup ([`#3081`](https://github.com/polybar/polybar/pull/3081)) by [@unclechu](https://github.com/unclechu)
 
 ## [3.7.1] - 2023-11-27
 ### Build

include/modules/backlight.hpp

@@ -67,7 +67,12 @@ namespace modules {
     brightness_handle m_val;
     brightness_handle m_max;
 
-    int m_percentage = 0;
+    /**
+     * Initial value set to a negative number so that any value read from the backlight file triggers an update during
+     * the first read.
+     * Otherwise, tokens may not be replaced
+     */
+    int m_percentage = -1;
 
     chrono::duration<double> m_interval{};
     chrono::steady_clock::time_point m_lastpoll;