bug#62675: 29.0.60; emoji-zoom-increase interacts badly with faces

[Robert Pluim <rpluim@gmail.com>]

--begin--
emacs -Q
C-p C-p
C-x 8 e + + + +
C-u C-x =
--end--

Visually, this does the right thing: the ';' has its height increased,
but the text properties look like this:

  face                 ((:height 1.1) font-lock-comment-delimiter-face :height 1.3310000000000004)
  fontified            t
  rear-nonsticky       t

Thatʼs not a valid 'face property, unless Iʼm misreading (info
"(elisp) Special Properties"), plus the ':height 1.331' bit is wrong,
thatʼs (expt 1.1 3), not (expt 1.1 4). Yet it works, the height has
been increased.

Is this a bug in our interpretation of 'face properties?

I can fix the code in `emoji-zoom-increase' to produce something like

((:height 1.4641000000000004) font-lock-comment-delimiter-face)

instead, but first Iʼd like to understand whatʼs going on.

In GNU Emacs 29.0.60 (build 15, x86_64-pc-linux-gnu, GTK+ Version
 3.24.24, cairo version 1.16.0) of 2023-03-14 built on rltb
Repository revision: 4b6f2a7028b91128934a19f83572f24106782225
Repository branch: emacs-29
Windowing system distributor 'The X.Org Foundation', version 11.0.12009000
System Description: Debian GNU/Linux 11 (bullseye)

Robert
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