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.

cmd/view-font/main.go

@@ -0,0 +1,101 @@
+package main
+
+import (
+	"flag"
+	"image"
+	"log"
+	"math"
+	"os"
+
+	"github.com/hajimehoshi/ebiten"
+
+	"code.ur.gs/lupine/ordoor/internal/assetstore"
+	"code.ur.gs/lupine/ordoor/internal/ui"
+)
+
+var (
+	gamePath = flag.String("game-path", "./orig", "Path to a WH40K: Chaos Gate installation")
+	fontName = flag.String("font", "", "Name of a font, e.g., basfont12")
+	txt      = flag.String("text", "Test string", "Text to render")
+
+	winX = flag.Int("win-x", 1280, "Pre-scaled window X dimension")
+	winY = flag.Int("win-y", 1024, "Pre-scaled window Y dimension")
+)
+
+type env struct {
+	font      *assetstore.Font
+	step      int
+	state     state
+	lastState state
+}
+
+type state struct {
+	zoom   float64
+	origin image.Point
+}
+
+func main() {
+	flag.Parse()
+
+	if *gamePath == "" || *fontName == "" {
+		flag.Usage()
+		os.Exit(1)
+	}
+
+	assets, err := assetstore.New(*gamePath)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	font, err := assets.Font(*fontName)
+	if err != nil {
+		log.Fatalf("Couldn't load font %s: %v", *fontName, err)
+	}
+
+	state := state{zoom: 8.0}
+	env := &env{
+		font:      font,
+		state:     state,
+		lastState: state,
+	}
+
+	win, err := ui.NewWindow(env, "View Font: "+*fontName, *winX, *winY)
+	if err != nil {
+		log.Fatal("Couldn't create window: %v", err)
+	}
+
+	win.OnMouseWheel(env.changeZoom)
+
+	// Main thread now belongs to ebiten
+	if err := win.Run(); err != nil {
+		log.Fatal(err)
+	}
+}
+
+func (e *env) Update(screenX, screenY int) error {
+	if e.step == 0 || e.lastState != e.state {
+		log.Printf("new state: zoom=%.2f", e.state.zoom)
+	}
+
+	e.step += 1
+	e.lastState = e.state
+
+	return nil
+}
+
+func (e *env) Draw(screen *ebiten.Image) error {
+	op := &ebiten.DrawImageOptions{}
+	op.GeoM.Scale(e.state.zoom, e.state.zoom) // apply current zoom factor
+
+	img, err := e.font.DrawLine(*txt)
+	if err != nil {
+		return err
+	}
+
+	return screen.DrawImage(img, op)
+}
+
+func (e *env) changeZoom(_, y float64) {
+	// Zoom in and out with the mouse wheel
+	e.state.zoom *= math.Pow(1.2, y)
+}