widget: make Icon honour its constraints

This is a breaking change as Icon.Layout no longer requests a size.

Before:
  sz := unit.Dp(20)
  ic.Layout(gtx, sz)

After:
  sz := gtx.Metric.Px(unit.Dp(20))
  gtx.Constraints.Min = image.Pt(sz, 0)
  ic.Layout(gtx)

Fixes gio#240

Signed-off-by: pierre <pierre.curto@gmail.com>

widget/material/button.go

@@ -170,7 +170,8 @@ func (b IconButtonStyle) Layout(gtx layout.Context) layout.Dimensions {
 				size := gtx.Px(b.Size)
 				if b.Icon != nil {
 					b.Icon.Color = b.Color
-					b.Icon.Layout(gtx, unit.Px(float32(size)))
+					gtx.Constraints.Min = image.Point{X: size}
+					b.Icon.Layout(gtx)
 				}
 				return layout.Dimensions{
 					Size: image.Point{X: size, Y: size},