d2renderers: d2fonts: add bold + italic styles for Source Code Pro

👋 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>

d2renderers/d2fonts/d2fonts.go

@@ -78,6 +78,12 @@ var sourceSansProItalicBase64 string
 //go:embed encoded/SourceCodePro-Regular.txt
 var sourceCodeProRegularBase64 string
 
+//go:embed encoded/SourceCodePro-Bold.txt
+var sourceCodeProBoldBase64 string
+
+//go:embed encoded/SourceCodePro-Italic.txt
+var sourceCodeProItalicBase64 string
+
 //go:embed encoded/ArchitectsDaughter-Regular.txt
 var architectsDaughterRegularBase64 string
 
@@ -108,6 +114,14 @@ func init() {
 			Family: SourceCodePro,
 			Style:  FONT_STYLE_REGULAR,
 		}: sourceCodeProRegularBase64,
+		{
+			Family: SourceCodePro,
+			Style:  FONT_STYLE_BOLD,
+		}: sourceCodeProBoldBase64,
+		{
+			Family: SourceCodePro,
+			Style:  FONT_STYLE_ITALIC,
+		}: sourceCodeProItalicBase64,
 		{
 			Family: HandDrawn,
 			Style:  FONT_STYLE_REGULAR,
@@ -144,6 +158,22 @@ func init() {
 		Family: SourceCodePro,
 		Style:  FONT_STYLE_REGULAR,
 	}] = b
+	b, err = fontFacesFS.ReadFile("ttf/SourceCodePro-Bold.ttf")
+	if err != nil {
+		panic(err)
+	}
+	FontFaces[Font{
+		Family: SourceCodePro,
+		Style:  FONT_STYLE_BOLD,
+	}] = b
+	b, err = fontFacesFS.ReadFile("ttf/SourceCodePro-Italic.ttf")
+	if err != nil {
+		panic(err)
+	}
+	FontFaces[Font{
+		Family: SourceCodePro,
+		Style:  FONT_STYLE_ITALIC,
+	}] = b
 	b, err = fontFacesFS.ReadFile("ttf/SourceSansPro-Bold.ttf")
 	if err != nil {
 		panic(err)

lib/textmeasure/textmeasure.go

@@ -4,6 +4,7 @@
 package textmeasure
 
 import (
+	"fmt"
 	"math"
 	"unicode"
 	"unicode/utf8"
@@ -138,7 +139,12 @@ func NewRuler() (*Ruler, error) {
 func (r *Ruler) addFontSize(font d2fonts.Font) {
 	sizeless := font
 	sizeless.Size = 0
-	face := truetype.NewFace(r.ttfs[sizeless], &truetype.Options{
+	ttf := r.ttfs[sizeless]
+	if ttf == nil {
+		panic(fmt.Errorf("sizeless font %s not found with style %s", font.Family, font.Style))
+	}
+
+	face := truetype.NewFace(ttf, &truetype.Options{
 		Size: float64(font.Size),
 	})
 	atlas := NewAtlas(face, ASCII)