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>

internal/f32color/rgba_test.go

@@ -0,0 +1,38 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+package f32color
+
+import (
+	"image/color"
+	"testing"
+)
+
+func TestNRGBAToRGBA_PostAlpha_Boundary(t *testing.T) {
+	for col := 0; col <= 0xFF; col++ {
+		for alpha := 0; alpha <= 0xFF; alpha++ {
+			in := color.NRGBA{R: uint8(col), A: uint8(alpha)}
+			premul := NRGBAToRGBA_PostAlpha(in)
+			if premul.A != uint8(alpha) {
+				t.Errorf("%v: got %v expected %v", in, premul.A, alpha)
+			}
+			if premul.R > premul.A {
+				t.Errorf("%v: R=%v > A=%v", in, premul.R, premul.A)
+			}
+		}
+	}
+}
+
+func TestNRGBAToLinearRGBA_Boundary(t *testing.T) {
+	for col := 0; col <= 0xFF; col++ {
+		for alpha := 0; alpha <= 0xFF; alpha++ {
+			in := color.NRGBA{R: uint8(col), A: uint8(alpha)}
+			premul := NRGBAToLinearRGBA(in)
+			if premul.A != uint8(alpha) {
+				t.Errorf("%v: got %v expected %v", in, premul.A, alpha)
+			}
+			if premul.R > premul.A {
+				t.Errorf("%v: R=%v > A=%v", in, premul.R, premul.A)
+			}
+		}
+	}
+}