Fix legacy effect's delay scaling and precision

Usually the delay parameter of legacy effects is scaled
to be relative to the PlayRes canvas. This happens explicitly
in VSFilter and automatically in libass.
However, this scaling in VSFilter happens _before_ applying
max(delay, 1) which means, if e.g. delay=0 it ends up as
1 ms per _storage pixel_. To get the same effect in libass we
must explicitly "unscale" the fallback for small or negative delays.

VSFilter also casts the scaled delay value to int afterwards,
which can lead to noticeable differences if the scaled value isn't
an integer. To emulate this in libass we do not want delay
to be an int (which would also ruin the unscaling for delay=0),
but we need to convert our already PlayRes-relative value to
a storage-relative one, then cast to int and finally convert
back to a PlayRes-relative value.

This rounding error can already be observed after just one second
for PlayResX=StorageX/8 and delay=25.

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