Refactor: Further improved Str_Copy() and its apidoc

The apidoc for Str_Copy() now very explicitly states what the function does.
Modified the copy behavior so that for regular strings, the source string's
buffer is duplicated exactly (entire reserved size) while static strings will get
new memory allocated for them.

doomsday/engine/api/dd_string.h

@@ -216,9 +216,20 @@ boolean Str_IsEmpty(const ddstring_t* ds);
 char* Str_Text(const ddstring_t* ds);
 
 /**
- * Makes a true copy.
+ * Makes a copy of @a src and replaces the previous contents of @a dest with
+ * it. The copy will have least as much memory reserved in its internal buffer
+ * as the original.
+ *
+ * If @a src is a static string (i.e., no memory allocated for its buffer), new
+ * memory will be allocated for the copy.
+ *
+ * @param dest  String where the copy is stored.
+ * @param src   Original string to copy.
+ *
+ * @return  The @a dest string with the copied content.
  */
 ddstring_t* Str_Copy(ddstring_t* dest, const ddstring_t* src);
+
 ddstring_t* Str_CopyOrClear(ddstring_t* dest, const ddstring_t* src);
 
 /**

doomsday/engine/portable/src/m_string.c

@@ -449,9 +449,21 @@ ddstring_t* Str_Copy(ddstring_t* str, const ddstring_t* other)
 #endif
         return str;
     }
-    allocateString(str, other->length, false);
-    strcpy(str->str, other->str);
-    str->length = other->length;
+    if(!other->size)
+    {
+        // The original string has no memory allocated; it's a static string.
+        allocateString(str, other->length, false);
+        strcpy(str->str, other->str);
+        str->length = other->length;
+    }
+    else
+    {
+        // Duplicate the other string's buffer in its entirety.
+        str->str = str->memAlloc(other->size);
+        memcpy(str->str, other->str, other->size);
+        str->size = other->size;
+        str->length = other->length;
+    }
     return str;
 }