all: use color.NRGBA in public API

color.RGBA has two problems with regards to using it.

First the color values need to be premultiplied, whereas most APIs
have non-premultiplied values. This is mainly to preserve color components
with low alpha values.

Second there are two ways to premultiply with sRGB. One is to premultiply
after sRGB conversion, the other is before. This makes using the API more
confusing.

Using color.NRGBA in sRGB makes it align with CSS.e

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

widget/icon.go

@@ -7,6 +7,7 @@ import (
 	"image/color"
 	"image/draw"
 
+	"gioui.org/internal/f32color"
 	"gioui.org/layout"
 	"gioui.org/op/paint"
 	"gioui.org/unit"
@@ -14,12 +15,12 @@ import (
 )
 
 type Icon struct {
-	Color color.RGBA
+	Color color.NRGBA
 	src   []byte
 	// Cached values.
 	op       paint.ImageOp
 	imgSize  int
-	imgColor color.RGBA
+	imgColor color.NRGBA
 }
 
 // NewIcon returns a new Icon from IconVG data.
@@ -28,7 +29,7 @@ func NewIcon(data []byte) (*Icon, error) {
 	if err != nil {
 		return nil, err
 	}
-	return &Icon{src: data, Color: color.RGBA{A: 0xff}}, nil
+	return &Icon{src: data, Color: color.NRGBA{A: 0xff}}, nil
 }
 
 func (ic *Icon) Layout(gtx layout.Context, sz unit.Value) layout.Dimensions {
@@ -49,7 +50,7 @@ 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] = ic.Color
+	m.Palette[0] = f32color.NRGBAToRGBA(ic.Color)
 	iconvg.Decode(&ico, ic.src, &iconvg.DecodeOptions{
 		Palette: &m.Palette,
 	})