Ded Reader|Added: Thing defs now support the Mods directive

The 'Mods' directive allows the properties of an existing definition
to be modified instead of creating a new one. This functionality can
be used to "patch" the original definitions, changing only some of
their properties.

Also added the alternative name "Mobj" for this definition type, for
consistency with the rest of the public API.

doomsday/engine/portable/src/def_read.c

@@ -868,13 +868,47 @@ static int DED_ReadData(ded_t* ded, const char* buffer, const char* _sourceFile)
             }
         }
 
-        if(ISTOKEN("Thing"))
+        if(ISTOKEN("Mobj") || ISTOKEN("Thing"))
         {
-            ded_mobj_t*         mo;
+            boolean bModify = false;
+            ded_mobj_t* mo, dummyMo;
+
+            ReadToken();
+            if(!ISTOKEN("Mods"))
+            {
+                // A new mobj type.
+                idx = DED_AddMobj(ded, "");
+                mo = &ded->mobjs[idx];
+            }
+            else if(!bCopyNext)
+            {
+                ded_mobjid_t otherMobjId;
+
+                READSTR(otherMobjId);
+                ReadToken();
 
-            // A new thing.
-            idx = DED_AddMobj(ded, "");
-            mo = &ded->mobjs[idx];
+                idx = Def_GetMobjNum(otherMobjId);
+                if(idx < 0)
+                {
+                    VERBOSE( Con_Message("Warning: Unknown Mobj %s in %s on line #%i, will be ignored.\n",
+                                         otherMobjId, source ? source->fileName : "?", source ? source->lineNumber : 0) )
+
+                    // We'll read into a dummy definition.
+                    memset(&dummyMo, 0, sizeof(dummyMo));
+                    mo = &dummyMo;
+                }
+                else
+                {
+                    mo = &ded->mobjs[idx];
+                    bModify = true;
+                }
+            }
+            else
+            {
+                SetError("Cannot both Copy(Previous) and Modify.");
+                retVal = false;
+                goto ded_end_read;
+            }
 
             if(prevMobjDefIdx >= 0 && bCopyNext)
             {
@@ -886,8 +920,12 @@ static int DED_ReadData(ded_t* ded, const char* buffer, const char* _sourceFile)
             for(;;)
             {
                 READLABEL;
-                RV_STR("ID", mo->id)
-                RV_INT("DoomEd number", mo->doomEdNum)
+                // ID cannot be changed when modifying
+                if(!bModify && ISLABEL("ID"))
+                {
+                    READSTR(mo->id);
+                }
+                else RV_INT("DoomEd number", mo->doomEdNum)
                 RV_STR("Name", mo->name)
                 RV_STR("Spawn state", mo->states[SN_SPAWN])
                 RV_STR("See state", mo->states[SN_SEE])
@@ -921,7 +959,12 @@ static int DED_ReadData(ded_t* ded, const char* buffer, const char* _sourceFile)
                 RV_END
                 CHECKSC;
             }
-            prevMobjDefIdx = idx;
+
+            // If we did not read into a dummy update the previous index.
+            if(idx > 0)
+            {
+                prevMobjDefIdx = idx;
+            }
         }
 
         if(ISTOKEN("State"))