opengl: fix readbackAsync causing crashes on quit.

Fixes #2298.

src/modules/graphics/opengl/GraphicsReadback.cpp

@@ -119,6 +119,17 @@ void GraphicsReadback::update()
 	}
 }
 
+bool GraphicsReadback::loadVolatile()
+{
+	// Don't need to (re)create anything here, since the work is done in sync.fence().
+	return true;
+}
+
+void GraphicsReadback::unloadVolatile()
+{
+	sync.cleanup();
+}
+
 } // opengl
 } // graphics
 } // love

src/modules/graphics/opengl/GraphicsReadback.h

@@ -22,6 +22,7 @@
 
 // LOVE
 #include "graphics/GraphicsReadback.h"
+#include "graphics/Volatile.h"
 #include "FenceSync.h"
 #include "common/math.h"
 
@@ -32,7 +33,7 @@ namespace graphics
 namespace opengl
 {
 
-class GraphicsReadback final : public love::graphics::GraphicsReadback
+class GraphicsReadback final : public love::graphics::GraphicsReadback, public Volatile
 {
 public:
 
@@ -43,6 +44,9 @@ public:
 	void wait() override;
 	void update() override;
 
+	bool loadVolatile() override;
+	void unloadVolatile() override;
+
 private:
 
 	FenceSync sync;