vout_subpictures.c: Do not call the scaling module if the subpicture is using RGBA. The current way subpictures scaling and blending modules loading work is kind of flawed. It assumes that the module will be able to handle all the possible types of input chromas. This worked fine previously (since all modules allowing to scale/blend YUVP also handled YUVA), but doesn't now (the scaling module doesn't support RGBA. blending works fine).
vout_pictures.*: Add core support for RGBA pictures.