Fixes for linux and mesa

[stuaxo]

Hi, I've been getting these working on mesa on Linux - these drivers are a lot stricter than the commercial ones so some things don't work right away.

Main stuff:

• Use forwardslashes for paths (python should be fine with this in Windows).
• glfw window hints for 3.3 context have to come before window creation
• glew_wish -glGetStringi for extensions killed stuff, so I've temporarily commented it out.

I've got up to tutorial5, but it's not included as I spent about a day trying to understand why textures were just black before rewriting the texture part, so that's something you might not want to 'just merge'.

S