Allow subpixel movement for legacy effects
Allow subpixel movement for legacy effects

This won't change the movement speed, as the total movement
is recalculated every frame, so this shouldn't negatively affect
VSFilter compatibility; if anything this should slightly improve
compatibility since VSFilter works in a supersampled space.
In any case, it does improves visual quality.

astiob/libass,libass/libass,astiob/libass,libass/libass