all: replace InvalidateOp with InvalidateCmd command

Curiously, InvalidateCmd is probably the only command that is appropriate
to call during layout.

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

widget/material/button.go

@@ -258,7 +258,7 @@ func drawInk(gtx layout.Context, c widget.Press) {
 
 	// Animate only ended presses, and presses that are fading in.
 	if !c.End.IsZero() || sizet <= 1.0 {
-		op.InvalidateOp{}.Add(gtx.Ops)
+		gtx.Execute(op.InvalidateCmd{})
 	}
 
 	if sizet > 1.0 {