Scale non-mono icons to be at least as large as mono

[danielwe]

Description

For icons with the pa stretch rule, font_patcher currently scales icons up to fit the cell in monospace patching, but only scales down for non-monospace. This makes some icons significantly smaller with non-monospaced patching than with monospaced, which is rather unexpected. Case in point: the nd-md-alpha alphabetic icons are tiny in non-monospaced fonts, but substantial in monospaced.

With this PR, non-monospaced patching never makes smaller icons than monospaced. For pa, if monospaced patching would scale up, non-monospaced scales up by the same amount, but no more.

Requirements / Checklist

• Read the Contributing Guidelines
• I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan.
  Issue number where discussion took place: #xxx
• If this contains a font/glyph add its origin as background info below (e.g. URL)
• Verified the license of any newly added font, glyph, or glyph set. License is: xxx

What does this Pull Request (PR) do?

• Factors the logic for determining iconheight out into a separate function that takes an optional single_width parameter
• Uses this function to correctly calculate the scale factors that would be obtained in single_width when needed
• For pa icons that don't need downscaling, sets the scale factor to the maximum of 1 and the single width scale factor.

How should this be manually tested?

• Patch a font in both mono and non-mono
• Look at 󰬇 (nd-md-alpha_z) in both variants at the same font size
• With the previous patcher, you'll see something like this, where the larger icon is with the Mono variant
  
• With this PR, the icon will have the larger size in both variants

Any background context you can provide?

See explanation.

What are the relevant tickets (if any)?

NA.

Screenshots (if appropriate or helpful)

See above.

[danielwe]

I borked a force push from a shallow clone, so this PR died. Resubmitted as #1926.