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/icon_test.go

@@ -9,7 +9,7 @@ import (
 
 	"gioui.org/layout"
 	"gioui.org/op"
-	"gioui.org/unit"
+
 	"golang.org/x/exp/shiny/materialdesign/icons"
 )
 
@@ -26,5 +26,49 @@ func TestIcon_Alpha(t *testing.T) {
 		Constraints: layout.Exact(image.Pt(100, 100)),
 	}
 
-	_ = icon.Layout(gtx, unit.Sp(18))
+	_ = icon.Layout(gtx)
+}
+
+// TestWidgetConstraints tests that widgets returns dimensions within their constraints.
+func TestWidgetConstraints(t *testing.T) {
+	_cs := func(v ...layout.Constraints) []layout.Constraints { return v }
+	for _, tc := range []struct {
+		label       string
+		widget      layout.Widget
+		constraints []layout.Constraints
+	}{
+		{
+			label: "Icon",
+			widget: func(gtx layout.Context) layout.Dimensions {
+				ic, _ := NewIcon(icons.ToggleCheckBox)
+				return ic.Layout(gtx)
+			},
+			constraints: _cs(
+				layout.Constraints{
+					Min: image.Pt(20, 0),
+					Max: image.Pt(100, 100),
+				},
+				layout.Constraints{
+					Max: image.Pt(100, 100),
+				},
+			),
+		},
+	} {
+		t.Run(tc.label, func(t *testing.T) {
+			for _, cs := range tc.constraints {
+				gtx := layout.Context{
+					Constraints: cs,
+					Ops:         new(op.Ops),
+				}
+				dims := tc.widget(gtx)
+				csr := image.Rectangle{
+					Min: cs.Min,
+					Max: cs.Max,
+				}
+				if !dims.Size.In(csr) {
+					t.Errorf("dims size %v not within constraints %v", dims.Size, csr)
+				}
+			}
+		})
+	}
 }