Implement bevel line joins

[dd10-e]

#34
As you see, all works :

but, when i press "z", we can see that :

It's the same bug that Round join have. I will take some times for see what doesen't work. It's something related to sides i think.

[nical]

Looks good! the reason the rendering looks off when you press 'z' a lot is that the line width becomes negative and the shader starts pushing vertices in the opposite direction. This is not related to your bevel implementation, it's really just the demo which should prevent the width from being negative since it makes no sense.

Could you squash the three commits into one commit? After that we are good to merge.

[dd10-e]

Done! Also it will resolve this issue #165 normally.
Thanx for your explanations!

[nical]

Thanks!