Fix: [OpenGL] Main loop expects to start with the video buffer unmapped. (#9100)

src/video/sdl2_opengl_v.cpp

@@ -74,6 +74,8 @@ const char *VideoDriver_SDL_OpenGL::Start(const StringList &param)
 		this->Stop();
 		return "Can't get pointer to screen buffer";
 	}
+	/* Main loop expects to start with the buffer unmapped. */
+	this->ReleaseVideoPointer();
 
 	return nullptr;
 }