update hob description: facegroups

doc/file_hob_spec.txt

@@ -23,22 +23,29 @@ OF * object header 116B
 
 OF * facegroup header 
 {
-  2B number of facegroups? NOF
-  2B number of facegroups? NOF
-  facegoup definitions 
-  {
-    offsets, names, floats
+  2B number of facegroups NOF
+  2B NX ?
+  NOF * facegroup/meshdef0 offset {
+    4B int ?
+    4B int facegroup/meshdef0 offset
+  }
+  2B NOF again
+  2B NX ?  
+  NX * offsets + zeros
+  NOF * name {
+    2B facegroup number
+    8B char
   }
 
   4B 0xFFFFFFFF header end marker
 }
 
-NOF * facegroup/meshdef0 132B 
+NOF * facegroup/meshdef0 (fg/m0) 132B 
 {
-  4B int
-  4B int
-  4B int
-  4B int
+  4B int offset to next fg/m0, 0 if there's no next, or in some other circumstances
+  4B int offset to prev?
+  4B int offset to beginning if this is not first fg/m0?
+  4B int offset to end if offset to next = 0
   4B int offset to meshdef1 + 4
   8B zero
   48B 3 * {
@@ -56,7 +63,8 @@ NOF * facegroup/meshdef1 96B
   4B int facedef end offset
   20B zero
   4B int vertices used
-  8B zero
+  4B int ?
+  4B zero
   4B int face def offset
   4B int vertex def offset
   52B zero