swscale/utils: Use full chroma interpolation for rgb4/8 and dither none

Dither none is only implemented in full chroma interpolation for these rgb formats
Its also a obscure choice (producing less nice images) that implementing it in the
other code-paths makes no sense

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libswscale/utils.c

@@ -1330,7 +1330,7 @@ av_cold int sws_init_context(SwsContext *c, SwsFilter *srcFilter,
         if (c->dither == SWS_DITHER_AUTO)
             c->dither = (flags & SWS_FULL_CHR_H_INT) ? SWS_DITHER_ED : SWS_DITHER_BAYER;
         if (!(flags & SWS_FULL_CHR_H_INT)) {
-            if (c->dither == SWS_DITHER_ED || c->dither == SWS_DITHER_A_DITHER || c->dither == SWS_DITHER_X_DITHER) {
+            if (c->dither == SWS_DITHER_ED || c->dither == SWS_DITHER_A_DITHER || c->dither == SWS_DITHER_X_DITHER || c->dither == SWS_DITHER_NONE) {
                 av_log(c, AV_LOG_DEBUG,
                     "Desired dithering only supported in full chroma interpolation for destination format '%s'\n",
                     av_get_pix_fmt_name(dstFormat));