Polished wall generation, though work is needed good enough for main now

mask.go

@@ -41,6 +41,12 @@ func (m *PixelMask) SetXY(x, y int) {
 	}
 }
 
+func (m *PixelMask) ClearXY(x, y int) {
+	if m.InBounds(x, y) {
+		m.Data[m.index(x, y)] = 0
+	}
+}
+
 func (m *PixelMask) GetPoint(p image.Point) bool {
 	return m.GetXY(p.X, p.Y)
 }