Added preliminary support for the SDL2 touch api.
Missing items:
1. Timestamps
2. TouchID (the Device ID)

Additionally, mouse events are still sent out for games using the
mouse API.
By default, turned off mouse simulation from touch events.
Added workaround for FFOS which, when two fingers touch the
screen roughly simultaneously, sends a repeat 'touch start' event
for one of the fingers.
This filters it out.
Added support for passing touch events with a deviceID of
`SDL_TOUCH_MOUSEID`.
With this in place, games can rely solely on touch events and they
will also work on systems using mouse input.
Formatting change. Removed commented-out code.
Updated the C structures corresponding to those events to contain
the `which` field whose value will be `SDL_TOUCH_MOUSEID` when
it is a simulated mouse event.
Reactivated SDL simulating mouse events from touch events.
Include the SDL2 Copyright notice on the headers that contain SDL2
content.
Fixed SDL_PeepEvents.
Fixed event interception in Browser.calculateMouseEvent
Added comment explaining the need for the `SDL.lastTouch` property.
Removed the `SDL.lastTouch` field which used to be used to provide
the last coordinates when simulating a mouse-up event.
We can instead use the coordinates from the touch in the
`changedTouches` property to send the coordinates.
Merge branch 'sdl-touch3' of https://github.com/michaeljbishop/emscripten into new-beta2

Conflicts:
	src/library_sdl.js