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.

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