avfilter/vf_scale: tag output color space

When using vf_scale to force a specific output color space, also tag
this on the AVFrame. (Mirroring existing logic for output range)

Move the sanity fix for RGB after the new assignment, to avoid leaking
bogus YUV colorspace metadata for RGB spaces.

libavfilter/vf_scale.c

@@ -799,18 +799,6 @@ scale:
     out->width  = outlink->w;
     out->height = outlink->h;
 
-    // Sanity checks:
-    //   1. If the output is RGB, set the matrix coefficients to RGB.
-    //   2. If the output is not RGB and we've got the RGB/XYZ (identity)
-    //      matrix configured, unset the matrix.
-    //   In theory these should be in swscale itself as the AVFrame
-    //   based API gets in, so that not every swscale API user has
-    //   to go through duplicating such sanity checks.
-    if (av_pix_fmt_desc_get(out->format)->flags & AV_PIX_FMT_FLAG_RGB)
-        out->colorspace = AVCOL_SPC_RGB;
-    else if (out->colorspace == AVCOL_SPC_RGB)
-        out->colorspace = AVCOL_SPC_UNSPECIFIED;
-
     if (scale->output_is_pal)
         avpriv_set_systematic_pal2((uint32_t*)out->data[1], outlink->format == AV_PIX_FMT_PAL8 ? AV_PIX_FMT_BGR8 : outlink->format);
 
@@ -857,8 +845,22 @@ scale:
                                      brightness, contrast, saturation);
 
         out->color_range = out_full ? AVCOL_RANGE_JPEG : AVCOL_RANGE_MPEG;
+        if (scale->out_color_matrix != AVCOL_SPC_UNSPECIFIED)
+            out->colorspace = scale->out_color_matrix;
     }
 
+    // Sanity checks:
+    //   1. If the output is RGB, set the matrix coefficients to RGB.
+    //   2. If the output is not RGB and we've got the RGB/XYZ (identity)
+    //      matrix configured, unset the matrix.
+    //   In theory these should be in swscale itself as the AVFrame
+    //   based API gets in, so that not every swscale API user has
+    //   to go through duplicating such sanity checks.
+    if (av_pix_fmt_desc_get(out->format)->flags & AV_PIX_FMT_FLAG_RGB)
+        out->colorspace = AVCOL_SPC_RGB;
+    else if (out->colorspace == AVCOL_SPC_RGB)
+        out->colorspace = AVCOL_SPC_UNSPECIFIED;
+
     av_reduce(&out->sample_aspect_ratio.num, &out->sample_aspect_ratio.den,
               (int64_t)in->sample_aspect_ratio.num * outlink->h * link->w,
               (int64_t)in->sample_aspect_ratio.den * outlink->w * link->h,