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/switch.go

@@ -19,8 +19,8 @@ import (
 
 type SwitchStyle struct {
 	Color struct {
-		Enabled  color.RGBA
-		Disabled color.RGBA
+		Enabled  color.NRGBA
+		Disabled color.NRGBA
 	}
 	Switch *widget.Bool
 }
@@ -124,7 +124,7 @@ func (s SwitchStyle) Layout(gtx layout.Context) layout.Dimensions {
 	return layout.Dimensions{Size: dims}
 }
 
-func drawDisc(ops *op.Ops, sz float32, col color.RGBA) {
+func drawDisc(ops *op.Ops, sz float32, col color.NRGBA) {
 	defer op.Push(ops).Pop()
 	rr := sz / 2
 	r := f32.Rectangle{Max: f32.Point{X: sz, Y: sz}}