[FIXED] allow negative mouse coordinates also with Mouse.next()

[Matthias]

Current the VM parameter "org.lwjgl.input.Mouse.allowNegativeMouseCoords" only affects Mouse.poll().

This patch does 2 things:
a) It implements this behavior also for Mouse.next()
b) The internal variable is renamed to "clipMouseCoordinatesToWindow" and a getter & setter is added.

The existing VM parameter is used to initialize this field as before - so no behavior changes when this feature is not used.