New issue by ashiire on mblaze repository

https://github.com/leahneukirchen/mblaze/issues/247

Description:
When `mshow` renders a message, the numbering of structure markers differs from that shown by its list mode. This becomes a problem when browsing messages (i.e. using `mless`) and wanting to extract a specific attachment. 

The discrepancy seems to arise from parts not rendered due to multipart/alternative preferences, as they are neither mentioned in the structure markers, nor counted. Perhaps they should be both mentioned and counted but not rendered?

Structure as rendered:
```
--- 1: multipart/related size=27924 ---
--- --- 2: multipart/alternative size=11924 ---
--- --- --- 3: text/plain size=885 ---

[...]

--- --- 4: image/jpeg size=11547 name="image001.jpg" ---
```
Structure as listed:
```
  1: multipart/related size=27924
    2: multipart/alternative size=11924
      3: text/plain size=885
      4: text/html size=9819
    5: image/jpeg size=11547 name="image001.jpg"
```