bug#50798: 28.0.50; Tab line close button is off-center until it is highlighted with the mouse

[Po Lu]

The close button for each tab in a tab line is off-center unless it's
under mouse face.

Enable tab-line-mode, and compare the position of the close button (X)
in any tab when it is under mouse face to when it is not.  The
difference is quite obvious.

In GNU Emacs 28.0.50 (build 3, x86_64-pc-linux-gnu, X toolkit, cairo version 
1.16.0, Xaw scroll bars)
 of 2021-09-25 built on chomper
Windowing system distributor 'The X.Org Foundation', version 11.0.12101002
System Description: Fedora 34 (Workstation Edition)

Configured using:
 'configure --with-native-compilation --with-x-toolkit=lucid
 --with-imagemagick'

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GSETTINGS HARFBUZZ
IMAGEMAGICK JPEG JSON LCMS2 LIBSELINUX LIBSYSTEMD LIBXML2 MODULES
NATIVE_COMP NOTIFY INOTIFY PDUMPER PNG RSVG SOUND THREADS TIFF
TOOLKIT_SCROLL_BARS X11 XDBE XIM XPM LUCID ZLIB