[Rd] Tight bounding box around text in graphics?

[Duncan Murdoch]

I've mentioned in previous messages that I'm trying to redo rgl text.

Part of what I need is to measure the size of strings in pixels when 
they are drawn by base graphics.

It appears that

   strwidth(texts, "user", cex = cex, font = font, family = family)

gives accurate measurements of the width in user coordinates.  I've got 
those set up to match pixels, so I'm fine here.

However, the equivalent call for strheight() only measures height above 
the baseline according to the docs, and indeed the number is smaller 
than the size of what's displayed.  Descenders (e.g. the tail of "y") 
aren't counted.

Is there a way to measure how far a character might descend?  Is it 
valid to assume it won't descend more than a line height below the top 
of the char?

I have a partial solution -- textshaping::shape_text gives a "height" 
value that includes lots of space below the character, and a 
"top_border" value that measures from the top of the textbox to the 
baseline.  So I think `height - top_border` would give me what I'm 
asking for.  But this only works with graphics devices in the ragg 
package.  Is there a general solution?

Duncan Murdoch