restorable: Ignore the error when restoring failed due to being not ready

(driver.Graphics).BeginFrame tries to restore the images, but
the context might be lost at that time yet. If the attempt to
restore the context because the driver is not ready, return
silently.

Fixes #1133

internal/driver/graphics.go

@@ -15,6 +15,8 @@
 package driver
 
 import (
+	"errors"
+
 	"github.com/hajimehoshi/ebiten/internal/affine"
 	"github.com/hajimehoshi/ebiten/internal/thread"
 )
@@ -37,6 +39,9 @@ type Graphics interface {
 	MaxImageSize() int
 }
 
+// GraphicsNotReady represents that the graphics driver is not ready for recovering from the context lost.
+var GraphicsNotReady = errors.New("graphics not ready")
+
 type Image interface {
 	Dispose()
 	IsInvalidated() bool

internal/graphicsdriver/opengl/context_js.go

@@ -212,7 +212,7 @@ func (c *context) reset() error {
 	c.gl = js.Value{}
 	c.ensureGL()
 	if c.gl.Call("isContextLost").Bool() {
-		return fmt.Errorf("opengl: the context is lost")
+		return driver.GraphicsNotReady
 	}
 	gl := c.gl
 	gl.Call("enable", blend)

internal/restorable/images.go

@@ -17,6 +17,7 @@ package restorable
 import (
 	"path/filepath"
 
+	"github.com/hajimehoshi/ebiten/internal/driver"
 	"github.com/hajimehoshi/ebiten/internal/graphicscommand"
 )
 
@@ -90,7 +91,11 @@ func RestoreIfNeeded() error {
 		}
 	}
 
-	if err := graphicscommand.ResetGraphicsDriverState(); err != nil {
+	err := graphicscommand.ResetGraphicsDriverState()
+	if err == driver.GraphicsNotReady {
+		return nil
+	}
+	if err != nil {
 		return err
 	}
 	return theImages.restore()