shareable: Bug fix: mutex should not be used at finalizers

Updates #913

internal/restorable/images.go

@@ -56,10 +56,8 @@ var theImages = &images{
 //
 // ResolveStaleImages is intended to be called at the end of a frame.
 func ResolveStaleImages() {
-	defer func() {
-		// Until the begin o the frame (by RestoreIfNeeded, any operations are deferred.
-		theImages.m.Lock()
-	}()
+	// Until the begin o the frame (by RestoreIfNeeded, any operations are locked.
+	defer theImages.m.Lock()
 
 	graphicscommand.FlushCommands()
 	if !needsRestoring() {

internal/shareable/shareable.go

@@ -73,9 +73,6 @@ func init() {
 }
 
 func resolveDeferred() {
-	deferredM.Lock()
-	defer deferredM.Unlock()
-
 	for _, f := range deferred {
 		f()
 	}
@@ -151,8 +148,7 @@ var (
 
 	imagesToMakeShared = map[*Image]struct{}{}
 
-	deferred  []func()
-	deferredM sync.Mutex
+	deferred []func()
 )
 
 // isShareable reports whether the new allocation can use the shareable backends.
@@ -388,9 +384,7 @@ func (i *Image) at(x, y int) (byte, byte, byte, byte) {
 // A function from finalizer must not be blocked, but disposing operation can be blocked.
 // Defer this operation until it becomes safe. (#913)
 func (i *Image) disposeFromFinalizer() {
-	deferredM.Lock()
-	defer deferredM.Unlock()
-
+	// deferred doesn't have to be, and should not be protected by a mutex.
 	deferred = append(deferred, func() {
 		i.dispose(true)
 	})