Added dash lines example
Merge branch 'master' of https://github.com/vispy/vispy

* 'master' of https://github.com/vispy/vispy:
  Look, no app.use()!
  Fixes in vertex shader of line_collection.
  Remobed unused method.
  The app.use function is now smart It can be called without arguments or simply not be called at all!
  Somehow these didnt make it in the last commit... Also added not about _vispy_set_current: I think the backends should call it right before the emit the paint event.
  Made delta be 1.0 and -1.0 for scroll events. Also some small tweaks/docs in the backends.