Fixed multitouch emulation in sdl2

~~(Not for merging yet, as below)~~

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, so this shouldn't be merged quite yet.~~