shareable: Use color vertex values instead of ColorM

This is an optimization: using ColorM might cause graphics command
separation.

internal/shareable/shareable.go

@@ -257,6 +257,17 @@ func (i *Image) DrawImage(img *Image, vertices []float32, indices []uint16, colo
 // Do not call Fill on emptyImage or the program causes infinite recursion.
 var emptyImage = NewImage(16, 16)
 
+func init() {
+	// Initialize emptyImage. As emptyImage is used for Fill, emptyImage itself cannot be initialized with Fill.
+	// Call ReplacePixels instead.
+	w, h := emptyImage.Size()
+	pix := make([]byte, 4*w*h)
+	for i := range pix {
+		pix[i] = 0xff
+	}
+	emptyImage.ReplacePixels(pix)
+}
+
 // Fill fills the image with a color. This affects not only the (0, 0)-(width, height) region but also the whole
 // framebuffer region.
 func (i *Image) Fill(r, g, b, a uint8) {
@@ -267,22 +278,25 @@ func (i *Image) Fill(r, g, b, a uint8) {
 	i.ensureNotShared()
 	backendsM.Unlock()
 
+	rf := float32(0)
+	gf := float32(0)
+	bf := float32(0)
+	af := float32(0)
+	if a > 0 {
+		rf = float32(r) / float32(a)
+		gf = float32(g) / float32(a)
+		bf = float32(b) / float32(a)
+		af = float32(a) / 0xff
+	}
+
 	dw, dh := i.backend.restorable.SizePowerOf2()
 	sw, sh := emptyImage.Size()
 	vs := emptyImage.QuadVertices(0, 0, sw, sh,
 		float32(dw)/float32(sw), 0, 0, float32(dh)/float32(sh), 0, 0,
-		1, 1, 1, 1)
+		rf, gf, bf, af)
 	is := graphics.QuadIndices()
 
-	var colorm *affine.ColorM
+	i.DrawImage(emptyImage, vs, is, nil, graphics.CompositeModeCopy, graphics.FilterNearest, graphics.AddressClampToZero)
-	if a > 0 {
-		rf := float32(r) / float32(a)
-		gf := float32(g) / float32(a)
-		bf := float32(b) / float32(a)
-		af := float32(a) / 0xff
-		colorm = colorm.Translate(rf, gf, bf, af)
-	}
-	i.DrawImage(emptyImage, vs, is, colorm, graphics.CompositeModeCopy, graphics.FilterNearest, graphics.AddressClampToZero)
 }
 
 func (i *Image) ReplacePixels(p []byte) {