internal/graphicscommand: bug fix: IsInvalidated was not concurrent-safe

This function was not called actually, so this is not a real problem.
However, this could be a potential problem for a future GLES driver (#292).

Updates #292
Closes #2321

internal/graphicscommand/command.go

@@ -625,6 +625,20 @@ func (c *newShaderCommand) Exec(graphicsDriver graphicsdriver.Graphics, indexOff
 	return nil
 }
 
+type isInvalidatedCommand struct {
+	result bool
+	image  *Image
+}
+
+func (c *isInvalidatedCommand) String() string {
+	return fmt.Sprintf("is-invalidated: image: %d", c.image.id)
+}
+
+func (c *isInvalidatedCommand) Exec(graphicsDriver graphicsdriver.Graphics, indexOffset int) error {
+	c.result = c.image.image.IsInvalidated()
+	return nil
+}
+
 // InitializeGraphicsDriverState initialize the current graphics driver state.
 func InitializeGraphicsDriverState(graphicsDriver graphicsdriver.Graphics) (err error) {
 	runOnRenderingThread(func() {

internal/graphicscommand/image.go

@@ -181,18 +181,26 @@ func (i *Image) WritePixels(pixels []byte, x, y, width, height int) {
 	})
 }
 
-func (i *Image) IsInvalidated() bool {
+func (i *Image) IsInvalidated(graphicsDriver graphicsdriver.Graphics) (bool, error) {
 	if i.screen {
 		// The screen image might not have a texture, and in this case it is impossible to detect whether
 		// the image is invalidated or not.
-		panic("graphicscommand: IsInvalidated cannot be called on the screen image")
+		return false, fmt.Errorf("graphicscommand: IsInvalidated cannot be called on the screen image")
 	}
 
 	// i.image can be nil before initializing.
 	if i.image == nil {
-		return false
+		return false, nil
 	}
-	return i.image.IsInvalidated()
+
+	c := &isInvalidatedCommand{
+		image: i,
+	}
+	theCommandQueue.Enqueue(c)
+	if err := theCommandQueue.Flush(graphicsDriver); err != nil {
+		return false, err
+	}
+	return c.result, nil
 }
 
 func (i *Image) dumpName(path string) string {

internal/restorable/image.go

@@ -661,11 +661,8 @@ func (i *Image) Dispose() {
 //
 // If an image is invalidated, GL context is lost and all the images should be restored asap.
 func (i *Image) isInvalidated(graphicsDriver graphicsdriver.Graphics) (bool, error) {
-	// FlushCommands is required because c.offscreen.impl might not have an actual texture.
-	if err := graphicscommand.FlushCommands(graphicsDriver); err != nil {
-		return false, err
-	}
-	return i.image.IsInvalidated(), nil
+	// IsInvalidated flushes the commands internally.
+	return i.image.IsInvalidated(graphicsDriver)
 }
 
 func (i *Image) Dump(graphicsDriver graphicsdriver.Graphics, path string, blackbg bool, rect image.Rectangle) (string, error) {