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/material/button.go

@@ -22,10 +22,10 @@ import (
 type ButtonStyle struct {
 	Text string
 	// Color is the text color.
-	Color        color.RGBA
+	Color        color.NRGBA
 	Font         text.Font
 	TextSize     unit.Value
-	Background   color.RGBA
+	Background   color.NRGBA
 	CornerRadius unit.Value
 	Inset        layout.Inset
 	Button       *widget.Clickable
@@ -33,15 +33,15 @@ type ButtonStyle struct {
 }
 
 type ButtonLayoutStyle struct {
-	Background   color.RGBA
+	Background   color.NRGBA
 	CornerRadius unit.Value
 	Button       *widget.Clickable
 }
 
 type IconButtonStyle struct {
-	Background color.RGBA
+	Background color.NRGBA
 	// Color is the icon color.
-	Color color.RGBA
+	Color color.NRGBA
 	Icon  *widget.Icon
 	// Size is the icon size.
 	Size   unit.Value
@@ -272,7 +272,7 @@ func drawInk(gtx layout.Context, c widget.Press) {
 	const col = 0.8
 	ba, bc := byte(alpha*0xff), byte(col*0xff)
 	defer op.Push(gtx.Ops).Pop()
-	rgba := f32color.MulAlpha(color.RGBA{A: 0xff, R: bc, G: bc, B: bc}, ba)
+	rgba := f32color.MulAlpha(color.NRGBA{A: 0xff, R: bc, G: bc, B: bc}, ba)
 	ink := paint.ColorOp{Color: rgba}
 	ink.Add(gtx.Ops)
 	rr := size * .5