Draft: Support replacing font families for scriptfont and scriptscriptfont

[zauguin]

READ ME FIRST: Please understand that in most cases we will not be able to merge a pull request because there are a lot of internal activities needed when updating the LaTeX2e sources. If you have a code suggestion please discuss it with the team first.

In LaTeX's math model scripts use the same fonts as the regular math font, but in a smaller size. Similarly double subscripts use an even smaller version of the same font. Sometimes it makes sense to use special adaptions to make fonts work better in scripts. In Computer Modern this is handled by Optical Sizes, but this has the side effect that it only works for certain main text sizes.
In OpenType the concept of script font adjustments is separate from optical sizes and therefore does trigger many edge cases.
Therefore this extends NFSS's script font handling by allowing to register replacement font families to use instead of the base math font when used in a subscript or sub-subscript.

Internal housekeeping

Status of pull request

• Feedback wanted
• Under development

Checklist of required changes before merge will be approved

• Test file(s) added
• Version and date string updated in changed source files
• Relevant \changes entries in source included
• Relevant changes.txt updated
• Rollback provided (if necessary)?
• ltnewsX.tex (and/or latexchanges.tex) updated

[davidcarlisle]

Looks good, obviously v?.? would need fixing. probably also a short note in ltnews.