[Bug] Capacities in vehicle replace window don't increase beyond original

[CK]

The capacities (both passenger, mail and goods) of a convoy do not increase beyond the capacity of the original (to be replaced) convoy in the replace vehicle window. It's a cosmetic bug but nonetheless a bit annoying.

Steps to reproduce:

• build a train of a couple of cars
• start the train
• open the vehicle replace window and do something that would increase capacity
• Shown capacity will not increase beyond the original convoy's capacity

[[C] Ranran]

https://github.com/jamespetts/simutrans-extended/blob/7e076b469ad6c5e8e330f4f1d98478a20c26aa40/gui/components/gui_convoy_assembler.cc#L788
cnv-> get_vehicle_count() in this line does not return the expected value in the replace_frame case.

This seems to be due to counting taking into account the class changes applied in the class manager for the purpose of counting capacity with the code below, if so, in the case of a replace frame, I think we need to add code to count the capacity based on vehicles(vehicle_desc_t *desc).
There may be a smarter way, but at least the current code behaves as reported as a bug. (´・ω・｀)