restorable: Bug fix: Reading pixels from a volatile image might fail

Fixes #793

internal/restorable/image.go

@@ -241,7 +241,7 @@ func (i *Image) DrawImage(img *Image, vertices []float32, indices []uint16, colo
 	}
 	theImages.makeStaleIfDependingOn(i)
 
-	if img.stale || img.volatile || i.screen || !IsRestoringEnabled() {
+	if img.stale || img.volatile || i.screen || !IsRestoringEnabled() || i.volatile {
 		i.makeStale()
 	} else {
 		i.appendDrawImageHistory(img, vertices, indices, colorm, mode, filter, address)
@@ -274,7 +274,7 @@ func (i *Image) appendDrawImageHistory(image *Image, vertices []float32, indices
 }
 
 func (i *Image) readPixelsFromGPUIfNeeded() {
-	if i.basePixels == nil || i.drawImageHistory != nil || i.stale {
+	if i.basePixels == nil || len(i.drawImageHistory) > 0 || i.stale {
 		graphicscommand.FlushCommands()
 		i.readPixelsFromGPU()
 		i.drawImageHistory = nil

internal/restorable/images_test.go

@@ -652,3 +652,27 @@ func TestReplacePixelsOnly(t *testing.T) {
 }
 
 // TODO: How about volatile/screen images?
+
+// Issue #793
+func TestReadPixelsFromVolatileImage(t *testing.T) {
+	const w, h = 16, 16
+	dst := NewImage(w, h, true)
+	src := NewImage(w, h, false)
+
+	// First, make sure that dst has pixels
+	dst.ReplacePixels(make([]byte, 4*w*h), 0, 0, w, h)
+
+	// Second, draw src to dst. If the implementation is correct, drawImageHistory is created.
+	fill(src, 0xff, 0xff, 0xff, 0xff)
+	vs := graphics.QuadVertices(w, h, 0, 0, 1, 1, 1, 0, 0, 1, 0, 0, 1, 1, 1, 1)
+	is := graphics.QuadIndices()
+	dst.DrawImage(src, vs, is, nil, graphics.CompositeModeCopy, graphics.FilterNearest, graphics.AddressClampToZero)
+
+	// Read the pixels. If the implementation is correct, dst tries to read its pixels from GPU due to
+	// drawImageHistory items.
+	want := byte(0xff)
+	got, _, _, _ := dst.At(0, 0)
+	if got != want {
+		t.Errorf("got: %v, want: %v", got, want)
+	}
+}