Reset screen joystick buttons/axes on initialization. Do not send mouse events from a screen joystick under touch emulation, as those would be re-interpreted as further touch events. Minor Input code cleanup.
Document the touch emulation feature briefly.
Add touch emulation stationary fingers for multi-gestures by pressing down other mouse buttons than the left.
Remove redundant check from Input code.
For Travis CI - switch CI build to use OS X build environment.