swscale: provide a default scaler if none is set

Lanczos for general case, sinc for upscaling, Gaussian for
downscaling. According to current literature these scalers
should be the best quality-wise algorithms for each case.

Inspired from a patch by wm4 <nfxjfg@googlemail.com>
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libswscale/utils.c

@@ -914,7 +914,17 @@ av_cold int sws_init_context(SwsContext *c, SwsFilter *srcFilter,
                  SWS_SINC          |
                  SWS_SPLINE        |
                  SWS_BICUBLIN);
-    if (!i || (i & (i - 1))) {
+
+    /* provide a default scaler if not set by caller */
+    if (!i) {
+        if (dstW < srcW && dstH < srcH)
+            flags |= SWS_GAUSS;
+        else if (dstW > srcW && dstH > srcH)
+            flags |= SWS_SINC;
+        else
+            flags |= SWS_LANCZOS;
+        c->flags = flags;
+    } else if (i & (i - 1)) {
         av_log(c, AV_LOG_ERROR,
                "Exactly one scaler algorithm must be chosen\n");
         return AVERROR(EINVAL);