Add a field for user IDs

  // This field is kept and returned as-is throughout the shaping process.
  // When you are using the context API, it contains a codepoint index always!
  // To get the original user ID with the context API, you need to get the corresponding kbts_shape_codepoint
  // with kbts_ShapeGetShapeCodepoint(Context, Glyph->UserIdOrCodepointIndex, ...);
  int UserIdOrCodepointIndex;

!!! WARNING !!!
When using the context API, UserIdOrCodepointIndex will _always_ be a
codepoint index. To get your original user ID, you need to do:

kbts_shape_codepoint ShapeCodepoint;
kbts_ShapeGetShapeCodepoint(Context, Glyph->UserIdOrCodepointIndex, &ShapeCodepoint);
int MyUserId = ShapeCodepoint.UserId;