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/draw_test.go

@@ -0,0 +1,21 @@
+package scenario
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestCARTtoISOtoCART(t *testing.T) {
+	for i := 0; i < 120; i++ {
+		for j := 0; j < 120; j++ {
+			orig := IsoPt{X: float64(i), Y: float64(j)}
+			asPix := orig.ToCart()
+			andBack := asPix.ToISO()
+
+			t.Logf("%v,%v: asPix = %v", i, j, asPix)
+
+			require.Equal(t, orig, andBack)
+		}
+	}
+}