wip: Subset fonts by fontstyle

[ShupingHe]

#1107

this pr helps reduce more font size of diagram

[ShupingHe]

below is the overall idea of this pr:
well in the previous optimization #1089 , we collect all the characters, called corpus, inside a diagram, and then subset our font by cutting the unnecssary fonts. what remains after this optimization is, say,if we only have a bold a character, we'd still include both regular or other fonts for this a character.
so, what we need to do is when collecting characters, we also need their style info, then when we subset our fonts, we can get rid of all unnecessary fonts.

[ShupingHe]

@alixander i asked you some questions in the related issue but got no responses... anyway, no worries

i draft this pr, can you help to take a glimpse, so i can know if i'm on the right direction

[alixander]

this looks like the right approach!

the e2e test deltas have some errors though

(btw to get these, i just run
git checkout master e2etests/testdata
and then
./ci/e2ereport.sh --delta
)

[ShupingHe]

@alixander for this script:

costumes: {
  shape: sql_table
  id: int {constraint: primary_key}
}

i notice that for the generated diagram, text consumes is bold. the text style is added here

d2/d2graph/d2graph.go

Line 571 in b08431b

isBold := !obj.IsContainer() && obj.Shape.Value != "text"

but inside the output svg, class text-bold can't be found, is that expected ? is it a bug?

[alixander]

@donglixiaoche ah, yeah i think that is a bug. it just didn't affect anything because Class has its own renderer that ignored the value set on d2target:

d2/d2renderers/d2svg/class.go

Line 14 in 91beb8d

func classHeader(diagramHash string, shape d2target.Shape, box *geo.Box, text string, textWidth, textHeight, fontSize float64) string {

[ShupingHe]

@alixander but basically we add bold font according to text-bold class, and after this optimization, if no text-bold is found, bold font won't be included. so the bug you mentioned above is caused by this bug.

do you think i need fix this bug here?

[alixander]

yes, i think it was just an innocuous bug until now, but this method will require that bugfix