Fix some errors in iso->pix->iso conversions, add debugging

Maps now render a bit more properly, and our mouse position can be
correctly assigned to a cell. Kind of, mostly.

internal/ui/window.go

@@ -35,6 +35,7 @@ type Window struct {
 	Title             string
 	KeyUpHandlers     map[ebiten.Key]func()
 	MouseWheelHandler func(float64, float64)
+	MouseClickHandler func()
 
 	// Allow the "game" to be switched out at any time
 	game Game
@@ -72,6 +73,10 @@ func (w *Window) OnMouseWheel(f func(x, y float64)) {
 	w.MouseWheelHandler = f
 }
 
+func (w *Window) OnMouseClick(f func()) {
+	w.MouseClickHandler = f
+}
+
 func (w *Window) Layout(_, _ int) (int, int) {
 	return w.xRes, w.yRes
 }
@@ -130,6 +135,12 @@ func (w *Window) Update(screen *ebiten.Image) (outErr error) {
 		}
 	}
 
+	if w.MouseClickHandler != nil {
+		if inpututil.IsMouseButtonJustPressed(ebiten.MouseButtonLeft) {
+			w.MouseClickHandler()
+		}
+	}
+
 	if ebiten.IsDrawingSkipped() {
 		return nil
 	}