internal/atlas: backends for unmanaged images were never released

Closes #3030

internal/atlas/export_test.go

@@ -69,3 +69,9 @@ func DeferredFuncCountForTesting() int {
 	defer deferredM.Unlock()
 	return len(deferred)
 }
+
+func BackendCountForTesting() int {
+	backendsM.Lock()
+	defer backendsM.Unlock()
+	return len(theBackends)
+}

internal/atlas/image.go

@@ -689,18 +689,14 @@ func (i *Image) deallocate() {
 		return
 	}
 
-	if !i.isOnAtlas() {
-		i.backend.image.Dispose()
-		i.backend.image = nil
-		return
-	}
-
-	i.backend.page.Free(i.node)
-	if !i.backend.page.IsEmpty() {
-		// As this part can be reused, this should be cleared explicitly.
-		r := i.regionWithPadding()
-		i.backend.clearPixels(r)
-		return
+	if i.isOnAtlas() {
+		i.backend.page.Free(i.node)
+		if !i.backend.page.IsEmpty() {
+			// As this part can be reused, this should be cleared explicitly.
+			r := i.regionWithPadding()
+			i.backend.clearPixels(r)
+			return
+		}
 	}
 
 	i.backend.image.Dispose()

internal/atlas/image_test.go

@@ -881,4 +881,26 @@ func TestGC(t *testing.T) {
 	}
 }
 
+func TestDallocateUnmanagedImageBackends(t *testing.T) {
+	const w, h = 16, 16
+	img0 := atlas.NewImage(w, h, atlas.ImageTypeUnmanaged)
+	img1 := atlas.NewImage(w, h, atlas.ImageTypeUnmanaged)
+
+	// Call DrawTriangles to ensure the images are on backends.
+	vs := quadVertices(w, h, 0, 0, 1)
+	is := graphics.QuadIndices()
+	dr := image.Rect(0, 0, w, h)
+	img0.DrawTriangles([graphics.ShaderSrcImageCount]*atlas.Image{img1}, vs, is, graphicsdriver.BlendCopy, dr, [graphics.ShaderSrcImageCount]image.Rectangle{}, atlas.NearestFilterShader, nil, graphicsdriver.FillRuleFillAll)
+
+	// Get the difference of the number of backends before and after the images are deallocated.
+	c := atlas.BackendCountForTesting()
+	img0.Deallocate()
+	img1.Deallocate()
+
+	diff := c - atlas.BackendCountForTesting()
+	if got, want := diff, 2; got != want {
+		t.Errorf("got: %d, want: %d", got, want)
+	}
+}
+
 // TODO: Add tests to extend image on an atlas out of the main loop