Revert "widget/material: propagate ButtonLayout minimum constraints to content"

This reverts commit 52ccc183b5.

Reason for revert:

This doesn't seem like a good idea after all. The reason for the change was to
propagate the minimum constraints to the button content. But in the simplest case,
a label, stretching the button will make the label stretch as well, leaving the label
top-aligned.

We'll revisit this issue if a real use-case comes up.

widget/material/button.go

@@ -109,9 +109,9 @@ func (b ButtonLayout) Layout(gtx *layout.Context, button *widget.Button, w layou
 			}
 		}),
 		layout.Stacked(func() {
+			gtx.Constraints.Width.Min = hmin
+			gtx.Constraints.Height.Min = vmin
 			layout.Center.Layout(gtx, func() {
-				gtx.Constraints.Width.Min = hmin
-				gtx.Constraints.Height.Min = vmin
 				b.Inset.Layout(gtx, func() {
 					paint.ColorOp{Color: b.Color}.Add(gtx.Ops)
 					w()