Fixed multitouch emulation in sdl2

Resolves https://github.com/kivy/kivy/issues/2507

I think this is the right way to do things, maintaining a list of current touches and not recording movement if no button is pressed (this seems to be what pygame did).

The issue isn't entirely resolved, ctrl+click doesn't work with the multitouch red dots, but I think this is a separate issue. I'm looking at it and will add to this pr if it's something simple.