use iterators for events

[ddkwork]

Signed-off-by: ddkwork

This change will improve the event traversal syntax

	for {
		ev, ok := gtx.Event(pointer.Filter{
			Target: r,
			Kinds:  pointer.Press | pointer.Release,
		})
		if !ok {
			break
		}
	  }

//------------------------------->

	for e := range gtx.Events(pointer.Filter{Target: r,Kinds:pointer.Press | pointer.Release}) {
             ....
	}

Using the range iteration will never traverse to the null pointer, and the return value bool will automatically stop traversing, so it is not necessary to judge bool downstream, and it will be automatically processed at the syntax level