Commit 7d36df315e restructured
draw_weapon_vclip, but unintentionally broke the spin of proximity bombs
because `modtime` could exceed `play_time * 2`, causing
`draw_vclip_object` to clamp `bitmapnum` to the last frame of the
animation. Switch from `-=` to `%=` so that modtime is always brought
back in range.
Reported-by: Descender1032 <https://forum.dxx-rebirth.com/showthread.php?tid=1005>
Fixes: 7d36df315e ("Simplify draw_weapon_vclip")