Lazily load sprite image data

This cuts memory use significantly, since many sprites in an object are
never used. We can get savings over time by evicting sprites when they
go out of scope, but that's, well, out of scope.

To achieve this, I introduce an assetstore package that is in charge of
loading things from the filesystem. This also allows some lingering
case-sensitivity issues to be handled cleanly.

I'd hoped that creating fewer ebiten.Image instances would help CPU
usage, but that doesn't seem to be the case.

internal/maps/maps.go

@@ -59,13 +59,17 @@ func (h Header) Height() int {
 	return MaxHeight
 }
 
-func (h Header) MapSetFilename() string {
+func (h Header) MapSetName() string {
 	idx := bytes.IndexByte(h.SetName[:], 0)
 	if idx < 0 {
 		idx = 8 // all 8 bytes are used
 	}
 
-	return string(h.SetName[0:idx:idx]) + ".set"
+	return string(h.SetName[0:idx:idx])
+}
+
+func (h Header) MapSetFilename() string {
+	return h.MapSetName() + ".set"
 }
 
 type ObjRef struct {