ui: introduce OpColor, a specialized OpImage for uniform colors

To avoid allocating an image.Image for OpImage.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

ui/draw/draw.go

@@ -5,6 +5,7 @@ package draw
 import (
 	"encoding/binary"
 	"image"
+	"image/color"
 	"math"
 
 	"gioui.org/ui"
@@ -18,6 +19,10 @@ type OpImage struct {
 	Rect image.Rectangle
 }
 
+type OpColor struct {
+	Col color.NRGBA
+}
+
 type OpDraw struct {
 	Rect f32.Rectangle
 }
@@ -57,6 +62,30 @@ func (i *OpImage) Decode(data []byte, refs []interface{}) {
 	}
 }
 
+func (c OpColor) Add(o *ui.Ops) {
+	data := make([]byte, ops.TypeColorLen)
+	data[0] = byte(ops.TypeColor)
+	data[1] = c.Col.R
+	data[2] = c.Col.G
+	data[3] = c.Col.B
+	data[4] = c.Col.A
+	o.Write(data)
+}
+
+func (c *OpColor) Decode(data []byte, refs []interface{}) {
+	if ops.OpType(data[0]) != ops.TypeColor {
+		panic("invalid op")
+	}
+	*c = OpColor{
+		Col: color.NRGBA{
+			R: data[1],
+			G: data[2],
+			B: data[3],
+			A: data[4],
+		},
+	}
+}
+
 func (d OpDraw) Add(o *ui.Ops) {
 	data := make([]byte, ops.TypeDrawLen)
 	data[0] = byte(ops.TypeDraw)