internal/processtest: improve test issue2815

WritePixels for 1x1 image is optimized at internal/ui, and Ebitengine
doesn't read pixels from GPU in this case. Enlarge the tested image,
and use DrawImage to ensure to invalidate the internal cache.

Updates #2815

internal/processtest/testdata/issue2815.go

@@ -37,14 +37,17 @@ func (g *Game) Update() error {
 		g.end0 = make(chan struct{})
 		g.end1 = make(chan struct{})
 		g.errCh = make(chan error)
-		img := ebiten.NewImage(1, 1)
+		src := ebiten.NewImage(1, 2)
-		img.WritePixels([]byte{0xff, 0xff, 0xff, 0xff})
+		src.WritePixels([]byte{0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff})
+		img := ebiten.NewImage(1, 2)
 		go func() {
 			t := time.Tick(time.Microsecond)
 		loop:
 			for {
 				select {
 				case <-t:
+					// Call DrawImage every time in order to invalidate the internal pixels cache.
+					img.DrawImage(src, nil)
 					got := img.At(0, 0).(color.RGBA)
 					want := color.RGBA{0xff, 0xff, 0xff, 0xff}
 					if got != want {

internal/ui/ui.go

@@ -130,15 +130,16 @@ func (u *UserInterface) readPixels(mipmap *mipmap.Mipmap, pixels []byte, region
 	if !ok {
 		ch := make(chan error)
 		u.context.appendDeferredFunc(func() {
+			defer close(ch)
 			ok, err := mipmap.ReadPixels(u.graphicsDriver, pixels, region)
+			if err != nil {
+				ch <- err
+				return
+			}
 			if !ok {
 				// This never reaches since this function must be called in a frame.
 				panic("ui: ReadPixels unexpectedly failed")
 			}
-			if err != nil {
-				ch <- err
-			}
-			close(ch)
 		})
 
 		// If this function is called from the game (Update/Draw) goroutine in between two frames,