internal/ui: recreate an offscreen image when isScreenClearedEveryFrame is toggled

This is a preparation to remove SetVolatile and SetIsolated in
the atlas package.

internal/ui/context.go

@@ -145,7 +145,18 @@ func (c *context) updateFrameImpl(graphicsDriver graphicsdriver.Graphics, update
 }
 
 func (c *context) drawGame(graphicsDriver graphicsdriver.Graphics) {
-	c.offscreen.mipmap.SetVolatile(theGlobalState.isScreenClearedEveryFrame())
+	if c.offscreen.volatile != theGlobalState.isScreenClearedEveryFrame() {
+		w, h := c.offscreen.width, c.offscreen.height
+		c.offscreen.MarkDisposed()
+		c.offscreen = c.game.NewOffscreenImage(w, h)
+
+		// TODO: Give volatile/isolated property to the constructor.
+		if theGlobalState.isScreenClearedEveryFrame() {
+			c.offscreen.setVolatile(true)
+		} else {
+			c.offscreen.mipmap.SetIsolated(true)
+		}
+	}
 
 	// Even though updateCount == 0, the offscreen is cleared and Draw is called.
 	// Draw should not update the game state and then the screen should not be updated without Update, but
@@ -245,12 +256,17 @@ func (c *context) layoutGame(outsideWidth, outsideHeight float64, deviceScaleFac
 	if c.offscreen == nil {
 		c.offscreen = c.game.NewOffscreenImage(ow, oh)
 
+		// TODO: Give volatile/isolated property to the constructor.
+		if theGlobalState.isScreenClearedEveryFrame() {
+			c.offscreen.setVolatile(true)
+		} else {
 			// Keep the offscreen an isolated image from an atlas (#1938).
 			// The shader program for the screen is special and doesn't work well with an image on an atlas.
 			// An image on an atlas is surrounded by a transparent edge,
 			// and the shader program unexpectedly picks the pixel on the edges.
 			c.offscreen.mipmap.SetIsolated(true)
 		}
+	}
 
 	return ow, oh
 }

internal/ui/image.go

@@ -33,6 +33,7 @@ type Image struct {
 	mipmap   *mipmap.Mipmap
 	width    int
 	height   int
+	volatile bool
 }
 
 func NewImage(width, height int) *Image {
@@ -51,6 +52,11 @@ func newScreenFramebufferImage(width, height int) *Image {
 	}
 }
 
+func (i *Image) setVolatile(volatile bool) {
+	i.volatile = volatile
+	i.mipmap.SetVolatile(volatile)
+}
+
 func (i *Image) MarkDisposed() {
 	if i.mipmap == nil {
 		return