NORSPEC-343 protect against edits that would set non-alpha faces to diffuse alpha mode blend

indra/newview/llpanelface.h

@@ -258,7 +258,8 @@ class LLPanelFace : public LLPanel
 					// _panel->isAlpha() "lies" when one face has alpha and the rest do not (NORSPEC-329)
 					// need to get per-face answer to this question for sane alpha mode retention on updates.
 					//					
-					U8 default_alpha_mode = object->isImageAlphaBlended(face) ? LLMaterial::DIFFUSE_ALPHA_MODE_BLEND : LLMaterial::DIFFUSE_ALPHA_MODE_NONE;
+					bool is_alpha_face = object->isImageAlphaBlended(face);
+					U8 default_alpha_mode = is_alpha_face ? LLMaterial::DIFFUSE_ALPHA_MODE_BLEND : LLMaterial::DIFFUSE_ALPHA_MODE_NONE;
 					
 					if (!current_material.isNull())
 					{
@@ -278,6 +279,11 @@ class LLPanelFace : public LLPanel
 					LLUUID	new_normal_map_id		= new_material->getNormalID();
 					LLUUID	new_spec_map_id		= new_material->getSpecularID();
 
+					if ((new_alpha_mode == LLMaterial::DIFFUSE_ALPHA_MODE_BLEND) && !is_alpha_face)
+					{
+						new_alpha_mode = LLMaterial::DIFFUSE_ALPHA_MODE_NONE;
+					}
+
 					bool is_default_blend_mode		= (new_alpha_mode == default_alpha_mode);
 					bool is_need_material			= !is_default_blend_mode || !new_normal_map_id.isNull() || !new_spec_map_id.isNull();