shouldn't clear the m_AttachmentClearValues inDeviceContextGLImpl::NextSubpass

https://github.com/DiligentGraphics/DiligentCore/blob/8be34c5ac4021eec223cc4609e79fa71421063f8/Graphics/GraphicsEngineOpenGL/src/DeviceContextGLImpl.cpp#L658
call m_AttachmentClearValues.clear() in  DeviceContextGLImpl::NextSubpass(),if there more than two subpass,the clear value not exists after first call for NextSubpass. delete this line work well.