skin_engine minor refactoring struct alignment No Functional Changes

Move some elements around to save 720 bytes in the skin engine with cabbie

saves some code manipulating the skin_helpers arrays in skin_engine
eliminate conditionals checking for pre/post process functions
using a dummy fn(), consolidate pre/post process into a single function
adding a bool preprocess to indicate stage

Change-Id: Id2df4706b73e9025c7300be135dc02e135e587fe

lib/skin_parser/skin_parser.h

@@ -107,15 +107,14 @@ struct skin_tag_parameter
         DEFAULT
     } type;
 
+    char type_code;
+
     union
     {
         int number;
         OFFSETTYPE(char*) text;
         OFFSETTYPE(struct skin_element*) code;
-    } data;
-
-    char type_code;
-            
+    } data;          
 };
 
 /* Defines an element of a SKIN file,
@@ -151,7 +150,7 @@ struct skin_element
 
 enum skin_cb_returnvalue
 {
-    CALLBACK_ERROR = -666,
+    CALLBACK_ERROR = -128,
     FEATURE_NOT_AVAILABLE,
     CALLBACK_OK = 0,
     /* > 0 reserved for future use */