Make a start on font rendering

I was hopeful I could use ebiten/text, but font.Face doesn't seem set
up for fixed-colour fonts.

internal/assetstore/font.go

@@ -0,0 +1,116 @@
+package assetstore
+
+import (
+	"fmt"
+	"log"
+
+	"github.com/hajimehoshi/ebiten"
+
+	"code.ur.gs/lupine/ordoor/internal/fonts"
+)
+
+type Font struct {
+	Name    string
+	mapping map[rune]*Sprite
+}
+
+func (a *AssetStore) Font(name string) (*Font, error) {
+	name = canonical(name)
+
+	// FIXME: these fonts don't exist. For now, point at one that does.
+	switch name {
+	case "imfnt13", "imfnt14":
+		name = "wh40k_12"
+	}
+
+	if font, ok := a.fonts[name]; ok {
+		return font, nil
+	}
+	log.Printf("Loading font %v", name)
+
+	filename, err := a.lookup(name, "fnt", "Fonts")
+	if err != nil {
+		return nil, err
+	}
+
+	raw, err := fonts.LoadFont(filename)
+	if err != nil {
+		return nil, err
+	}
+
+	objFile, err := a.lookup(raw.ObjectFile, "", "Fonts")
+	if err != nil {
+		return nil, err
+	}
+
+	obj, err := a.ObjectByPath(objFile)
+	if err != nil {
+		return nil, err
+	}
+
+	out := &Font{
+		Name:    name,
+		mapping: make(map[rune]*Sprite, len(raw.Mapping)),
+	}
+
+	for r, offset := range raw.Mapping {
+		spr, err := obj.Sprite(offset)
+		if err != nil {
+			return nil, err
+		}
+
+		out.mapping[r] = spr
+	}
+
+	a.fonts[name] = out
+
+	return out, nil
+}
+
+// FIXME: this violates the ebiten rules for fast drawing. We may need to do the
+// draw ourselves, with image.Paletted for each glyph to a single ebiten.Image
+//
+// FIXME: it'd be great if we didn't have to implement this all by ourselves;
+// golang.org/x/image/font and github.com/hajimehoshi/ebiten/text are *almost*
+// sufficient, but don't seem to like it when the glyphs are literal colours
+// instead of a mask.
+//
+// TODO: draw text in a bounding box, multiple lines, etc
+func (f *Font) DrawLine(text string) (*ebiten.Image, error) {
+	sprites := make([]*Sprite, 0, len(text))
+	width := 0
+	height := 0
+
+	for _, r := range text {
+		spr, ok := f.mapping[r]
+		if !ok {
+			return nil, fmt.Errorf("Font %v does not specify rune %v", f.Name, r)
+		}
+
+		width += spr.Rect.Dx()
+		if y := spr.Rect.Dy(); y > height {
+			height = y
+		}
+
+		sprites = append(sprites, spr)
+	}
+
+	img, err := ebiten.NewImage(width, height, ebiten.FilterDefault)
+	if err != nil {
+		return nil, err
+	}
+
+	xOff := 0
+	for _, spr := range sprites {
+		op := &ebiten.DrawImageOptions{}
+		op.GeoM.Translate(float64(xOff), 0)
+
+		xOff += spr.Rect.Dx()
+
+		if err := img.DrawImage(spr.Image, op); err != nil {
+			return nil, err
+		}
+	}
+
+	return img, nil
+}