--sub-margin-y is documented to scale with font size, which is scaled
"by" or "with" window depending on the options. However, when using
ass_set_font_scale to change font size, the y margin isn't scaled, so it is
still scaled to video size. This causes the y margin size becoming smaller
when the video becomes smaller, even when the font size isn't changing.
Fix this by also scale the MarginV with the font scale. Also use lrint
instead of round.
07acdf09b3