Re: `set-global-staff-size` fails within `\book`

[Werner LEMBERG]

>> Consider this snippet.
>> 
>> ```
>> \book {
>>   #(set-global-staff-size 20)
>>   { c' }
>> }
>> 
>> \book {
>>   #(set-global-staff-size 10)
>>   { c' }
>> }
>> ```

I played a bit more with this, and I've found that
`set-global-staff-size` affects the *next* book block, which is, well,
unexpected.

```
\book {
  \markup "30"
  #(set-global-staff-size 30)
  { c' }
}

\book {
  \markup "10"
  #(set-global-staff-size 10)
  { c' }
}

\book {
  \markup "10"
  #(set-global-staff-size 10)
  { c' }
}
```


    Werner