video_output: cleanup user Aspect Ratio handling

Extracted from !6042.

This MR ensures the vd->source->i_sar_num/i_sar_den properly reflects the user requested Aspect Ratio and that it's used by all display modules from there.

Some extra cleanup is added so that calling multiple controls in the same function just tells whether we need a reset or not (it's easier than assembling multiple VLC error values into one error values).