👋 I've been playing around with changing the entire font of a diagram to something fixed-width, starting by hand-editing `main.go`
I noticed that if I switched over the family to Source Code Pro, d2 crashed with the following stack:
```
[23:38:30] info: compiling GetUniqueColumnName-fix.d2...
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x1c4 pc=0x16b562c]
goroutine 43 [running]:
github.com/golang/freetype/truetype.(*Font).Bounds(0xc000669670?, 0x0?)
/Users/kevin/dev/go/pkg/mod/github.com/golang/freetype@v0.0.0-20170609003504-e2365dfdc4a0/truetype/truetype.go:378 +0xc
github.com/golang/freetype/truetype.NewFace(0x0, 0xc0003276c0)
/Users/kevin/dev/go/pkg/mod/github.com/golang/freetype@v0.0.0-20170609003504-e2365dfdc4a0/truetype/face.go:199 +0x325
oss.terrastruct.com/d2/lib/textmeasure.(*Ruler).addFontSize(0xc00037c000, {{0x1bb9a34, 0xd}, {0x1b82732, 0x4}, 0x1f})
/Users/kevin/dev/d2/lib/textmeasure/textmeasure.go:141 +0x112
oss.terrastruct.com/d2/lib/textmeasure.(*Ruler).MeasurePrecise(0xc00037c000, {{0x1bb9a34, 0xd}, {0x1b82732, 0x4}, 0x1f}, {0xc000582100, 0x1f})
/Users/kevin/dev/d2/lib/textmeasure/textmeasure.go:157 +0xde
oss.terrastruct.com/d2/lib/textmeasure.(*Ruler).Measure(...)
/Users/kevin/dev/d2/lib/textmeasure/textmeasure.go:151
oss.terrastruct.com/d2/d2graph.GetTextDimensions({0x0, 0x0, 0xc000657860?}, 0xc00037c000, 0xc000620280, 0xc000092020)
```
Which I tracked down to these missing styles, which appear to matter even if they aren't used.
I acquired the `ttf` files from https://fonts.google.com/specimen/Source+Code+Pro
I created the encoded fonts on my Mac with:
```
base64 -i SourceCodePro-Italic.ttf -o ../encoded/SourceCodePro-Italic.txt
```
Hopefully this is correct! Open to all feedback, especially since I think this is the first change I've ever made to a go program :)
Signed-off-by: Kevin David <kevin-david@github.com>
