Fix #82514: Default Value for Viewport Display Color's Alpha Wrong.

For now duplicate default diffuse color values into RNA code too. Sad
work-around, but fixes the problem, until we get rid of hacks like
'interpret 4 consecutive floats in DNA as an array of 4 floats in RNA'.

source/blender/makesdna/DNA_material_defaults.h

@@ -14,6 +14,7 @@
 /** \name Material Struct
  * \{ */
 
+/* Keep RGBA diffuse defaults in sync with #rna_def_material_display in rna_material.cc */
 #define _DNA_DEFAULT_Material \
   { \
     .r = 0.8, \

source/blender/makesrna/intern/rna_material.cc

@@ -453,6 +453,10 @@ static void rna_def_material_display(StructRNA *srna)
   RNA_def_property_array(prop, 4);
   RNA_def_property_override_flag(prop, PROPOVERRIDE_OVERRIDABLE_LIBRARY);
   RNA_def_property_ui_text(prop, "Diffuse Color", "Diffuse color of the material");
+  /* See #82514 for details, for now re-define defaults here. Keep in sync with
+   * #DNA_material_defaults.h */
+  static float diffuse_color_default[4] = {0.8f, 0.8f, 0.8f, 1.0f};
+  RNA_def_property_float_array_default(prop, diffuse_color_default);
   RNA_def_property_update(prop, 0, "rna_Material_draw_update");
 
   prop = RNA_def_property(srna, "specular_color", PROP_FLOAT, PROP_COLOR);