ass_parse: fix premature truncation on \t(\Xa)
ass_parse: fix premature truncation on \t(\Xa)

This caused animations to out-of-range alpha values to be clipped to the
uint8_t range _before_ performing the lerp, rather than only after.
VSFilter only truncates after. Using e.g. `\1a0\t(\1a1FF)` should run through
the entire 0-FF range _twice_.

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