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>

op/paint/doc.go

@@ -9,6 +9,6 @@ taking the current transformation into account.
 The current brush is set by either a ColorOp for a constant color, or
 ImageOp for an image, or LinearGradientOp for gradients.
 
-All color.RGBA values are in the sRGB color space.
+All color.NRGBA values are in the sRGB color space.
 */
 package paint

op/paint/paint.go

@@ -21,7 +21,7 @@ import (
 // See NewImageOp for details.
 type ImageOp struct {
 	uniform bool
-	color   color.RGBA
+	color   color.NRGBA
 	src     *image.RGBA
 
 	// handle is a key to uniquely identify this ImageOp
@@ -31,16 +31,16 @@ type ImageOp struct {
 
 // ColorOp sets the brush to a constant color.
 type ColorOp struct {
-	Color color.RGBA
+	Color color.NRGBA
 }
 
 // LinearGradientOp sets the brush to a gradient starting at stop1 with color1 and
 // ending at stop2 with color2.
 type LinearGradientOp struct {
 	Stop1  f32.Point
-	Color1 color.RGBA
+	Color1 color.NRGBA
 	Stop2  f32.Point
-	Color2 color.RGBA
+	Color2 color.NRGBA
 }
 
 // PaintOp fills fills the current clip area with the current brush.
@@ -58,7 +58,7 @@ type PaintOp struct {
 func NewImageOp(src image.Image) ImageOp {
 	switch src := src.(type) {
 	case *image.Uniform:
-		col := color.RGBAModel.Convert(src.C).(color.RGBA)
+		col := color.NRGBAModel.Convert(src.C).(color.NRGBA)
 		return ImageOp{
 			uniform: true,
 			color:   col,
@@ -138,7 +138,7 @@ func (d PaintOp) Add(o *op.Ops) {
 }
 
 // FillShape fills the clip shape with a color.
-func FillShape(ops *op.Ops, c color.RGBA, shape clip.Op) {
+func FillShape(ops *op.Ops, c color.NRGBA, shape clip.Op) {
 	defer op.Push(ops).Pop()
 	shape.Add(ops)
 	Fill(ops, c)
@@ -148,7 +148,7 @@ func FillShape(ops *op.Ops, c color.RGBA, shape clip.Op) {
 // is intended to be used with a clip.Op already in place to limit
 // the painted area. Use FillShape unless you need to paint several
 // times within the same clip.Op.
-func Fill(ops *op.Ops, c color.RGBA) {
+func Fill(ops *op.Ops, c color.NRGBA) {
 	defer op.Push(ops).Pop()
 	ColorOp{Color: c}.Add(ops)
 	PaintOp{}.Add(ops)