lavfi/drawutils: set subsampling for rgb too

Otherwise it is incorrectly set to 1.
Signed-off-by: Paul B Mahol <onemda@gmail.com>

libavfilter/drawutils.c

@@ -180,10 +180,8 @@ int ff_draw_init(FFDrawContext *draw, enum AVPixelFormat format, unsigned flags)
     draw->format    = format;
     draw->nb_planes = nb_planes;
     memcpy(draw->pixelstep, pixelstep, sizeof(draw->pixelstep));
-    if (nb_planes >= 3 && !(desc->flags & AV_PIX_FMT_FLAG_RGB)) {
     draw->hsub[1] = draw->hsub[2] = draw->hsub_max = desc->log2_chroma_w;
     draw->vsub[1] = draw->vsub[2] = draw->vsub_max = desc->log2_chroma_h;
-    }
     for (i = 0; i < ((desc->nb_components - 1) | 1); i++)
         draw->comp_mask[desc->comp[i].plane] |=
             1 << (desc->comp[i].offset_plus1 - 1);