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

src/video/opengl.cpp

@@ -1154,6 +1154,7 @@ void *OpenGLBackend::GetVideoBuffer()
 #endif
 
 	if (!this->persistent_mapping_supported) {
+		assert(this->vid_buffer == nullptr);
 		_glBindBuffer(GL_PIXEL_UNPACK_BUFFER, this->vid_pbo);
 		this->vid_buffer = _glMapBuffer(GL_PIXEL_UNPACK_BUFFER, GL_READ_WRITE);
 	} else if (this->vid_buffer == nullptr) {

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;
 }

src/video/win32_v.cpp

@@ -1311,6 +1311,8 @@ const char *VideoDriver_Win32OpenGL::Start(const StringList &param)
 		_cur_resolution = old_res;
 		return "Can't get pointer to screen buffer";
 	}
+	/* Main loop expects to start with the buffer unmapped. */
+	this->ReleaseVideoPointer();
 
 	MarkWholeScreenDirty();