ass_render_event: simplify clip conversion to screen coordinates
ass_render_event: simplify clip conversion to screen coordinates

The "depends on alignment" block is useless: if use_margins, then
the coordinates will be overridden, but if not, then all of the
various x/y2scr... calls delegate to x/y2scr_pos...

Rewrite several lines using FFMIN/FFMAX to keep them short.

ShiftMediaProject/libass,Akemi/libass,ShiftMediaProject/libass,xbmc/libass,Akemi/libass,libass/libass,xbmc/libass,astiob/libass,astiob/libass,libass/libass,MrSmile/libass,MrSmile/libass,Akemi/libass