Update README with references to package names, address one error

[sshine]

Hi there!

Thanks for providing this software! I enjoyed it many years ago; it might have been featured on Hacker News.

Recently a penpal referred to a picture that I had generated with this software, and it reminded me to revisit it.

It still works great; I needed to install a package and fix a permission problem.

In case anyone still stumbles upon this software, I thought these steps were easy for me but might not be easy for everyone.

---

When running on Ubuntu today, an error like

X Error of failed request:  BadValue (integer parameter out of range for operation)
  Major opcode of failed request:  151 (GLX)
  Minor opcode of failed request:  24 (X_GLXCreateNewContext)
  Value in failed request:  0x0
  Serial number of failed request:  35
  Current serial number in output stream:  36
name of display: :99

would occur.

This can be fixed by adding the current user to a system group called video.