widget: use correct color in Icon

iconvg seems to expect a linear premultiplied color.

Fixes gio#132

Signed-off-by: Egon Elbre <egonelbre@gmail.com>

widget/icon.go

@@ -51,7 +51,7 @@ func (ic *Icon) image(sz int) paint.ImageOp {
 	img := image.NewRGBA(image.Rectangle{Max: image.Point{X: sz, Y: int(float32(sz) * dy / dx)}})
 	var ico iconvg.Rasterizer
 	ico.SetDstImage(img, img.Bounds(), draw.Src)
-	m.Palette[0] = f32color.NRGBAToRGBA(ic.Color)
+	m.Palette[0] = f32color.NRGBAToLinearRGBA(ic.Color)
 	iconvg.Decode(&ico, ic.src, &iconvg.DecodeOptions{
 		Palette: &m.Palette,
 	})

widget/icon_test.go

@@ -0,0 +1,30 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+package widget
+
+import (
+	"image"
+	"image/color"
+	"testing"
+
+	"gioui.org/layout"
+	"gioui.org/op"
+	"gioui.org/unit"
+	"golang.org/x/exp/shiny/materialdesign/icons"
+)
+
+func TestIcon_Alpha(t *testing.T) {
+	icon, err := NewIcon(icons.ToggleCheckBox)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	icon.Color = color.NRGBA{B: 0xff, A: 0x40}
+
+	gtx := layout.Context{
+		Ops:         new(op.Ops),
+		Constraints: layout.Exact(image.Pt(100, 100)),
+	}
+
+	_ = icon.Layout(gtx, unit.Sp(18))
+}