color_frame: Fix #13887 Pixel display not updating on Image encoding change

A GimpColorFrame's GeglColor's Babl format's precision is set to the image's precision
when picked. On an Image encoding change to a different precision the colors are
perceptually identical, but for Sample Points set to 'Pixel' we need to run
gimp_color_frame_update after updating the frame's color to the higher precision one.

app/widgets/gimpcolorframe.c

@@ -617,6 +617,7 @@ gimp_color_frame_set_color (GimpColorFrame *frame,
       frame->sample_average                == sample_average                &&
       frame->x                             == x                             &&
       frame->y                             == y                             &&
+      gegl_color_get_format (frame->color) == gegl_color_get_format (color) &&
       gimp_color_is_perceptually_identical (frame->color, color))
     {
       g_clear_object (&frame->color);