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/assetstore.go

@@ -34,6 +34,7 @@ type AssetStore struct {
 	entries entryMap
 
 	// These members are used to store things we've already loaded
+	fonts   map[string]*Font
 	maps    map[string]*Map
 	menus   map[string]*Menu
 	objs    map[string]*Object
@@ -84,6 +85,7 @@ func (a *AssetStore) Refresh() error {
 
 	// Refresh
 	a.entries = newEntryMap
+	a.fonts = make(map[string]*Font)
 	a.maps = make(map[string]*Map)
 	a.menus = make(map[string]*Menu)
 	a.objs = make(map[string]*Object)