Rework spu scaling

Currently we always invoke spu_Create, which causes quite a few side effects (instantiating 2 video-converter, loading a text renderer module) even if no-spu is specified.

Moreover, we have a few assumptions that could/should be considered risky or invalid. Any module that advertises supporting YUVP->YUVA conversion will automatically be considered capable of handling YUVP->RGBA conversion.

Finally, this doesn't seem like the proper way if we intend to do subtitles scaling in hardware.

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