exception on older opengl version

[nczempin]

On an os that doesn't support the required opengl version, the game simply terminates with an error message on startup.

We should

• Find out exactly which OpenGL version is required
• Find out whether we can make any of the shaders optional and still have the game look pretty
• If it's too much effort to provide a shaderless version, exit more gracefully.

I can provide more details on the machine (it was Vista 32) some other time.

[PriorBlue]

I added a menu with different graphic options.

[nczempin]

I'm not sure if the options alone help avoid the exception. I haven't looked at exception handling in Lua, but if any of the settings fail (which are enabled by default) they should be automatically disabled.

Of course it would help if issue #55 were finished.