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/assetstore/map.go

@@ -77,6 +77,11 @@ func (m *Map) LoadSprites() error {
 	return nil
 }
 
+// FIXME: get rid of this
+func (m *Map) Cell(x, y, z int) maps.Cell {
+	return m.raw.Cells.At(x, y, z)
+}
+
 // SpritesForCell returns the sprites needed to correctly render this cell.
 // They should be rendered from first to last to get the correct ordering
 func (m *Map) SpritesForCell(x, y, z int) ([]*Sprite, error) {