Render SPU separately for snapshot
Currently, if a picture is getting snap-shot, the video output will force early SPU blending. That poses some problems:
- SPU are rendered ugly (at source rather than display resolution) for just the affected picture.
- OSD is disabled altogether for the affected picture.
- If SPU blending is not supported by the display, the normal blend filter will be destroyed and a snapshot blend filter is created instead. At the next picture, the same procedure is done in reverse.
Instead, we should render and blend the SPU separately for the snapshot picture.