Frustum culling documentation

[heinezen]

This PR adds several things related to frustum culling:

• [x] Documentation
  • [x] Frustum Culling
  • [x] Desription of all renderer demos, including screenshots
• [x] Demo 6 (frustum culling)
  • [x] Code comments
  • [x] Render different angles of sprites
  • [x] Draw the frame around the frustum borders to make it more obvious where the frustum is
• [x] Fix a bug in Frustum2d where the pixel size was not calculated correctly