Fix plotting of layer numbers greater six

Introduce 15 more colors for layers above six. For higher layers
the colors are repeated again.

I've also tried adding hatching to the layers to make the layer colors more unique, but it just does not look so nice.

Example text code for convenience.
```python
import gdsCAD.core
import gdsCAD.shapes


cell = gdsCAD.core.Cell('TEST')
for i, layer in enumerate(range(500)):
    cell.add(gdsCAD.core.Path([(0, i), (10, i)], 1, layer=layer))
    cell.add(gdsCAD.shapes.Rectangle((10, i), (20, i+1), layer=layer))
cell.show()
```