text_renderer: warn when the rendering cannot be done

Due to requested chromas not being supported.

modules/text_renderer/freetype/freetype.c

@@ -1154,6 +1154,9 @@ static subpicture_region_t *Render( filter_t *p_filter,
         break;
     }
 
+    if (region == NULL)
+        msg_Warn( p_filter, "no output chroma supported for rendering" );
+
 done:
     FreeLines( text_block.p_laid );
 

modules/text_renderer/svg.c

@@ -337,7 +337,10 @@ static subpicture_region_t *RenderText( filter_t *p_filter,
         if( p_chroma_list[i] == VLC_CODEC_BGRA )
             break;
         if( p_chroma_list[i] == 0 )
+        {
+            msg_Warn( p_filter, "no output chroma supported for rendering" );
             return NULL;
+        }
     }
 
     unsigned i_width = p_filter->fmt_out.video.i_visible_width;