avfilter/buffersrc: allow promoting color range to MPEG

Otherwise, passing an UNSPECIFIED frame to am MPEG-only filter graph
would trigger insertion of an unnecessary vf_scale filter, which would
perform a memcpy to convert between the two.

This is safe to do because unspecified YUV frames are already
universally assumed to be MPEG range, in particular by swscale.

libavfilter/buffersrc.c

@@ -506,8 +506,14 @@ static int query_formats(AVFilterContext *ctx)
             if ((ret = ff_add_format(&color_spaces, c->color_space)) < 0 ||
                 (ret = ff_set_common_color_spaces(ctx, color_spaces)) < 0)
                 return ret;
-            if ((ret = ff_add_format(&color_ranges, c->color_range)) < 0 ||
-                (ret = ff_set_common_color_ranges(ctx, color_ranges)) < 0)
+            if ((ret = ff_add_format(&color_ranges, c->color_range)) < 0)
+                return ret;
+            if (c->color_range == AVCOL_RANGE_UNSPECIFIED) {
+                /* allow implicitly promoting unspecified to mpeg */
+                if ((ret = ff_add_format(&color_ranges, AVCOL_RANGE_MPEG)) < 0)
+                    return ret;
+            }
+            if ((ret = ff_set_common_color_ranges(ctx, color_ranges)) < 0)
                 return ret;
         }
         break;