plugins/screencast: Use buffer format and modifier as specified in spa_video_info_raw

It appears like pipewire can ask kwin to allocate a drm buffer even though m_dmabufParams has been reset. It's underspecified when pipewire will start requesting new buffers after changing the stream params. So use the last reported video info to allocate buffers. If even it is wrong, then it's an issue somewhere in pipewire.