widget,widget/material: remove Color field from Icon

Icons are meant to be shared among multiple widgets, but their Color
state may end up with unexpected values after use. Replace the state
with and explicit argument to Layout.

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

widget/icon_test.go

@@ -19,14 +19,14 @@ func TestIcon_Alpha(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	icon.Color = color.NRGBA{B: 0xff, A: 0x40}
+	col := color.NRGBA{B: 0xff, A: 0x40}
 
 	gtx := layout.Context{
 		Ops:         new(op.Ops),
 		Constraints: layout.Exact(image.Pt(100, 100)),
 	}
 
-	_ = icon.Layout(gtx)
+	_ = icon.Layout(gtx, col)
 }
 
 // TestWidgetConstraints tests that widgets returns dimensions within their constraints.
@@ -41,7 +41,7 @@ func TestWidgetConstraints(t *testing.T) {
 			label: "Icon",
 			widget: func(gtx layout.Context) layout.Dimensions {
 				ic, _ := NewIcon(icons.ToggleCheckBox)
-				return ic.Layout(gtx)
+				return ic.Layout(gtx, color.NRGBA{A: 0xff})
 			},
 			constraints: _cs(
 				layout.Constraints{