Clarify gdk-pixbuf dependency purpose

swaybg is out-of-tree so not relevant here. swaybar's tray doesn't
actually depend on gdk-pixbuf, but gdk-pixbuf enables more image
formats for swaybar tray when available.

Closes: https://github.com/swaywm/sway/issues/7913

README.md

@@ -31,7 +31,7 @@ Install dependencies:
 * json-c
 * pango
 * cairo
-* gdk-pixbuf2 (optional: system tray)
+* gdk-pixbuf2 (optional: additional image formats for system tray)
 * [swaybg] (optional: wallpaper)
 * [scdoc] (optional: man pages) \*
 * git (optional: version info) \*

meson_options.txt

@@ -6,6 +6,6 @@ option('swaybar', type: 'boolean', value: true, description: 'Enable support for
 option('swaynag', type: 'boolean', value: true, description: 'Enable support for swaynag')
 option('xwayland', type: 'feature', value: 'auto', description: 'Enable support for X11 applications')
 option('tray', type: 'feature', value: 'auto', description: 'Enable support for swaybar tray')
-option('gdk-pixbuf', type: 'feature', value: 'auto', description: 'Enable support for more image formats in swaybg')
+option('gdk-pixbuf', type: 'feature', value: 'auto', description: 'Enable support for more image formats in swaybar tray')
 option('man-pages', type: 'feature', value: 'auto', description: 'Generate and install man pages')
 option('sd-bus-provider', type: 'combo', choices: ['auto', 'libsystemd', 'libelogind', 'basu'], value: 'auto', description: 'Provider of the sd-bus library')