libgimp: Use RGB/A u8 format for indexed color params

Resolves #11018.
Currently, babl formats are saved as strings when
passed as params. However, indexed palettes do not
use a "standard" encoding string but are created from
palettes (with a custom format). This results in an
error when we attempt to recreate the babl format from
the custom string encoding, as it doesn't exist at that
point.

This patch mitigates the problem by converting the indexed
color to RGB/A 8-bit when used in params. In the future when
indexed mode supports different color spaces and higher
precision, we will need to remove the hard coding. For now,
it solves the immediate problem.

libgimp/gimpgpparams-body.c

@@ -737,6 +737,7 @@ gimp_gp_param_to_value (gpointer        gimp,
             }
           g_object_unref (profile);
         }
+
       format = babl_format_with_space (encoding, space);
       color  = gegl_color_new ("black");
 
@@ -1064,6 +1065,29 @@ gimp_value_to_gp_param (const GValue *value,
       color  = g_value_get_object (value);
       format = gegl_color_get_format (color);
 
+      /* TODO: For indexed colors, we'll convert to R'G'B'(A) u8 as that
+       * is currently what we support. As indexed format support improves,
+       * we'll want to make this space and encoding agnostic. */
+      if (babl_format_is_palette (format))
+        {
+          const Babl *indexed_format = NULL;
+          gint        bpp;
+
+          bpp = babl_format_get_bytes_per_pixel (format);
+          if (bpp == 1)
+            indexed_format = babl_format_with_space ("R'G'B' u8",
+                                                     babl_format_get_space (format));
+          else if (bpp == 2)
+            indexed_format = babl_format_with_space ("R'G'B'A u8",
+                                                     babl_format_get_space (format));
+
+          /* TODO: This is to notify us in the future when indexed image can have more than
+           * 256 colors - we'll need to update encoding support accordingly */
+          g_return_if_fail (indexed_format != NULL);
+
+          format = indexed_format;
+        }
+
       param->data.d_gegl_color.size = babl_format_get_bytes_per_pixel (format);
       gegl_color_get_pixel (color, format, &param->data.d_gegl_color.data);