Start work on menu interactivity.

With this commit, we get a ui.Interface and ui.Widget type. The
interface monitors hover and mouse click state and tells the widgets
about them; the widgets execute code specified by the application when
events occur.

Next step: have wh40k load the main menu and play sound, etc.

internal/assetstore/menu.go

@@ -0,0 +1,81 @@
+package assetstore
+
+import (
+	"github.com/hajimehoshi/ebiten"
+
+	"code.ur.gs/lupine/ordoor/internal/menus"
+)
+
+type Menu struct {
+	assets *AssetStore
+	obj    *Object // TODO: handle multiple objects in the menu
+	raw    *menus.Menu
+
+	Name string
+}
+
+// FIXME: don't expose this
+func (m *Menu) Records() []*menus.Record {
+	return m.raw.Records
+}
+
+func (m *Menu) Images(start, count int) ([]*ebiten.Image, error) {
+	out := make([]*ebiten.Image, count)
+	for i := start; i < start+count; i++ {
+		sprite, err := m.Sprite(i)
+		if err != nil {
+			return nil, err
+		}
+
+		out[i-start] = sprite.Image
+	}
+
+	return out, nil
+}
+
+func (m *Menu) Sprite(idx int) (*Sprite, error) {
+	return m.obj.Sprite(idx)
+}
+
+func (a *AssetStore) Menu(name string) (*Menu, error) {
+	name = canonical(name)
+
+	if menu, ok := a.menus[name]; ok {
+		return menu, nil
+	}
+
+	filename, err := a.lookup(name, "mnu", "Menu")
+	if err != nil {
+		return nil, err
+	}
+
+	raw, err := menus.LoadMenu(filename)
+	if err != nil {
+		return nil, err
+	}
+
+	obj, err := a.loadMenuObject(raw) // TODO: multiple objects
+	if err != nil {
+		return nil, err
+	}
+
+	menu := &Menu{
+		assets: a,
+		obj:    obj,
+		raw:    raw,
+		Name:   name,
+	}
+
+	a.menus[name] = menu
+	return menu, nil
+}
+
+func (a *AssetStore) loadMenuObject(menu *menus.Menu) (*Object, error) {
+	filename := menu.ObjectFiles[0]
+	filename, err := a.lookup(filename, "", "Menu") // Extension already present
+	if err != nil {
+		return nil, err
+	}
+
+	return a.ObjectByPath(filename)
+}