internal/atlas: reland: clarify the logic when to update usedAsDestination

This is a reland of 2c9f5d9dad.

As the name is `usedAsDestination`, this should be updated whenever
the image is used as a rendering destination.

Confirmed that this change didn't cause a performance regression
like #2586.

Updates #2586
Updates #2676

internal/atlas/image.go

@@ -90,6 +90,10 @@ func putImagesOnSourceBackend(graphicsDriver graphicsdriver.Graphics) error {
 	for k := range imagesToPutOnSourceBackend {
 		delete(imagesToPutOnSourceBackend, k)
 	}
+
+	for i := range imagesBackendJustCreated {
+		delete(imagesBackendJustCreated, i)
+	}
 	return nil
 }
 
@@ -136,6 +140,8 @@ var (
 
 	imagesUsedAsDestination = map[*Image]struct{}{}
 
+	imagesBackendJustCreated = map[*Image]struct{}{}
+
 	deferred []func()
 
 	// deferredM is a mutex for the slice operations. This must not be used for other usages.
@@ -238,15 +244,21 @@ func (i *Image) ensureIsolatedFromSource(backends []*backend) {
 			bs = append(bs, b)
 		}
 		i.allocate(bs, false)
+		imagesBackendJustCreated[i] = struct{}{}
 		return
 	}
 
+	// imagesUsedAsDestination affects the counter usedAsDestination.
+	// The larger this counter is, the harder it is for the image to be transferred to the source backend.
+	// This counter is not updated when the backend is created in this frame.
+	if _, ok := imagesBackendJustCreated[i]; !ok {
+		imagesUsedAsDestination[i] = struct{}{}
+	}
+
 	if !i.isOnAtlas() {
 		return
 	}
 
-	imagesUsedAsDestination[i] = struct{}{}
-
 	// Check if i has the same backend as the given backends.
 	var needsIsolation bool
 	for _, b := range backends {

internal/atlas/image_test.go

@@ -676,8 +676,13 @@ func TestImageIsNotReputOnSourceBackendWithoutUsingAsSource(t *testing.T) {
 		Height: size,
 	}
 
-	// Use src2 as a rendering target, and make src2 an independent image.
-	src2.DrawTriangles([graphics.ShaderImageCount]*atlas.Image{src}, vs, is, graphicsdriver.BlendCopy, dr, graphicsdriver.Region{}, [graphics.ShaderImageCount - 1][2]float32{}, atlas.NearestFilterShader, nil, false)
+	// Use src2 as a rendering target, and make src2 on a destination backend.
+	//
+	// Call DrawTriangles multiple times.
+	// The number of DrawTriangles doesn't matter as long as these are called in one frame.
+	for i := 0; i < 2; i++ {
+		src2.DrawTriangles([graphics.ShaderImageCount]*atlas.Image{src}, vs, is, graphicsdriver.BlendCopy, dr, graphicsdriver.Region{}, [graphics.ShaderImageCount - 1][2]float32{}, atlas.NearestFilterShader, nil, false)
+	}
 	if got, want := src2.IsOnSourceBackendForTesting(), false; got != want {
 		t.Errorf("got: %v, want: %v", got, want)
 	}