bug#50424: 27.2; Tab bar button mouse face not clearing entirely

[Eli Zaretskii]

> From: Juri Linkov <juri@linkov.net>
> Date: Sat, 11 Sep 2021 22:43:44 +0300
> Cc: luangruo@yahoo.com, 50424@debbugs.gnu.org
> 
> > Thanks, I really appreciate your help.  After seeing the result
> > of your patch, one question that came up is whether it's possible
> > not to change the previously used dimensions?  With enlarged height
> > and width, now the buttons are not vertically aligned:
> 
> Sorry, actually this question was addressed to Eli
> for the commit with the same subject but different hash:
> db74a93659 that contains such changes:
> 
> - :margin (2 . 0)
> + :margin ,tab-bar-button-margin

This is to make the code do what the variable's documentation says.
Without that change, the images were sized using :margin '(2 . 0)',
but the C code used the value of tab-bar-button-margin, a simple
scalar, to place the image and clear its background.  The way to
affect the image dimensions is by changing the value of
tab-bar-button-margin, not by hard-coding the margin in the image
properties.