Fix mathtext rendering of \| and sizing of | and \| - #24239

Since we have to update the test image, maybe we can drop the extra \Vert and \vert at the end? At least, I think those are redundant with the first two.

I think the idea is to test \left and \right in combination with these. However, it doesn't really make sense for a single letter. I'll update the first to consider a fraction instead (to clearly illustrate a bug in | not being resized).

The new test images illustrates some of the issues, but others are missed. So the question is if we should also add tests for a single letter with \left/\right etc? For cm the bars are adjusted down in size for a.

Maybe @anntzer can weigh in on relevant test cases?

It seems like getting rid of the following line sorts out the sizing, but I have not checked what else it breaks...

I'll try to have a look, but can't do it right now.

Figured out a way to get the vertical bar size correct. #23250 (comment)

Added a second commit with that change so that one can see the differences. The rationale is to scale if there are no alternatives (or rather only a single alternative).

(Make sure to squash the commits.)

The drawback of the current approach of getting the size right is that the font size is changed, so wider bars, but probably still better than having too short ones...

Looks good to me; just need a squash.