text: use the first registered font as the default in Shaper

Before this change, the default font was the one registered with an empty
Typeface.

We're about to use the correct names for fonts, so change Shaper to
fallback to the first registered typeface instead.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

text/shaper.go

@@ -14,6 +14,7 @@ import (
 // Specify the default and fallback font by calling Register with the
 // empty Font.
 type Shaper struct {
+	def   Typeface
 	faces map[Font]*face
 }
 
@@ -25,6 +26,7 @@ type face struct {
 
 func (s *Shaper) Register(font Font, tf Face) {
 	if s.faces == nil {
+		s.def = font.Typeface
 		s.faces = make(map[Font]*face)
 	}
 	// Treat all font sizes equally.
@@ -72,7 +74,7 @@ func (s *Shaper) faceForFont(font Font) *face {
 	font.Size = unit.Value{}
 	tf := s.faceForStyle(font)
 	if tf == nil {
-		font.Typeface = ""
+		font.Typeface = s.def
 		tf = s.faceForStyle(font)
 	}
 	if tf == nil {