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.