Deserialize gamecode as packed in PackedGameDataTo (#738)

Deserialize gamecode as packed in PackedGameDataTo  (#738)

* Deserialize gamecode as packed in PackedGameDataTo

Most packets use a normal 32 bit int but not PackedGameDataTo. Instead,
Innersloth decided to waste a byte by using WritePacked. As v2 game
codes are always negative, this is less efficient, but alas.

It was documented in the pseudocode and I didn't spot the difference
when reviewing the implementation, so I made this mistake when
deduplicating this code.

Test plan:
- Start EHR on at least 2 clients
- Use the game mode "Natural Disasters"
- Observe that pixelated blobs are visible for all players, not just the host

* uopt: stop parsing code to string

* Address review comments