Add a fallback face set

internal/faces/fallback/face.go

@@ -0,0 +1,35 @@
+package fallbackFaces
+
+import "golang.org/x/image/font"
+import "git.tebibyte.media/tomo/tomo"
+import "golang.org/x/image/font/basicfont"
+import "git.tebibyte.media/tomo/backend/style"
+
+type faceSet struct {
+	regular    font.Face
+	bold       font.Face
+	italic     font.Face
+	boldItalic font.Face
+}
+
+// New creates a new fallback face set.
+func New () style.FaceSet {
+	// TODO maybe pre-generate different variations of this face
+	return &faceSet {
+		regular:    basicfont.Face7x13,
+		bold:       basicfont.Face7x13,
+		italic:     basicfont.Face7x13,
+		boldItalic: basicfont.Face7x13,
+	}
+}
+
+func (this *faceSet) Face (face tomo.Face) font.Face {
+	bold   := face.Weight >= 500
+	italic := face.Italic >= 0.1 || face.Slant >= 0.1
+	switch {
+	case bold && italic: return this.boldItalic
+	case bold:           return this.bold
+	case italic:         return this.italic
+	default:             return this.regular
+	}
+}