avfilter/vf_interlace: do not interlace already interlaced frames

The filter used to work this way before it was merged into tinterlace.
Signed-off-by: Marton Balint <cus@passwd.hu>

libavfilter/vf_tinterlace.c

@@ -529,6 +529,7 @@ static int init_interlace(AVFilterContext *ctx)
     if (tinterlace->mode <= MODE_BFF)
         tinterlace->mode += MODE_INTERLEAVE_TOP;
 
+    tinterlace->flags |= TINTERLACE_FLAG_BYPASS_IL;
     if (tinterlace->lowpass == VLPF_LIN)
         tinterlace->flags |= TINTERLACE_FLAG_VLPF;
     if (tinterlace->lowpass == VLPF_CMP)