layout: replace NewContext with a Queue argument to Context.Reset

We're about to reduce the scope of the Window.Queue by moving it
to FrameEvent. As a consequence, Context can no longer rely on a
Queue constant over its lifetime.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

layout/stack_test.go

@@ -9,7 +9,7 @@ import (
 
 func TestStack(t *testing.T) {
 	var gtx Context
-	gtx.Reset(nil, image.Point{X: 100, Y: 100})
+	gtx.Reset(nil, nil, image.Point{X: 100, Y: 100})
 	gtx.Constraints.Width.Min = 0
 	gtx.Constraints.Height.Min = 0
 	exp := image.Point{X: 60, Y: 70}