bug#52129: 29.0.50; Wish: Extend `:align-to center' to work on lines in buffer

[Arthur Miller]

I am not sure if the title is descriptive enough, or even correct, but as seen
from attached code and image, I am trying to align two strings in a center of
each strings line.

It works now when string-pixel-width correctly calculates pixel widths. However,
I would prefer if I could just use :align-to center as a spec for a string as
property. Currently we have to use space as a spacer as we used to use
gif-spacers back in time.

I am not sure about how semantics would work, there are several cases to
consider, but generally if a string or a stretch of text in a string were given
text property 'display '(:align-to center), then Emacs would align that stretch
of text in the middle of the line that text occupies.

There would be cases where text dooes not fit the line, where there are multiple
such substring in same line maybe etc.

I don't know if it is possible or not, I am not familar with the display code,
nor am I sure about the syntax of proposal, but it would be nice and probably
useful to have some more declarative approach then what I had to do procedurally
in the attached example.

aligned-clock.png
Description: PNG image

emacs-vision-clock.el
Description: Text document

In GNU Emacs 29.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.30, cairo 
version 1.17.4)
 of 2021-11-26 built on pascal
Repository revision: c32e8bdc23214793ffcb065ba8478570679c1f0a
Repository branch: alpha-patch
Windowing system distributor 'The X.Org Foundation', version 11.0.12101001
System Description: Arch Linux

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