Always set the swap interval on GLX

[jrmuizel]

This matches the behaviour of the WGL backend and allows
toggling vsync in situations where the swap interval defaults
to 1

[tomaka]

The problem is that "system default" should be a valid option as well.

This has been something that I've wanted to change for some time. In my opinion a better change is that the vsync option should be an enum with three variants (enabled, disabled, system default).

[jrmuizel]

So when I first tested this I must have made a mistake because it doesn't actually work. 0 is not a valid value to pass to SwapIntervalSGI and the mesa drivers don't support SwapIntervalExt. The only way to disable vsync with mesa is through SwapIntervalMESA.