event/size: add an Event.Size method.

This is similar to the Event.Bounds method. Some functions in other
packages take an image.Rectangle (i.e. bounds), others take an
image.Point (i.e size).

Change-Id: Ie892a8902992d4e6b9edfbf6d12342297fae4068
Reviewed-on: https://go-review.googlesource.com/14003
Reviewed-by: David Crawshaw <crawshaw@golang.org>

event/size/size.go

@@ -36,6 +36,12 @@ type Event struct {
 	Orientation Orientation
 }
 
+// Size returns the window's size in pixels, at the time this size event was
+// sent.
+func (e *Event) Size() image.Point {
+	return image.Point{e.WidthPx, e.HeightPx}
+}
+
 // Bounds returns the window's bounds in pixels, at the time this size event
 // was sent.
 //