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/scenario/manage.go

@@ -0,0 +1,25 @@
+package scenario
+
+import (
+	//	"image"
+
+	"github.com/hajimehoshi/ebiten"
+
+	"code.ur.gs/lupine/ordoor/internal/maps"
+)
+
+type CellPoint struct {
+	IsoPt
+	Z int
+}
+
+func (s *Scenario) CellAtCursor() (maps.Cell, CellPoint) {
+	x, y := ebiten.CursorPosition()
+	screenPos := CartPt{X: float64(s.Viewpoint.X + x), Y: float64(s.Viewpoint.Y + y)}
+	isoPos := screenPos.ToISO()
+	// Convert to cell coordinates.
+	// TODO: zoom support will need a camera
+	// FIXME: adjust for Z level
+
+	return s.area.Cell(int(isoPos.X), int(isoPos.Y), 0), CellPoint{IsoPt: isoPos, Z: s.ZIdx}
+}