PROTOCOL: Update to gameplay protocol extensions.

[dsvensson]

Aligns QTV to currently enabled gameplay extensions:

• PEXT_SPAWNSTATIC2 - Delta spawns for extended attributes
  • svc_fte_spawnstatic2
  • svc_fte_spawnbaseline2
• PEXT_ENTITYDBL - Up to 1024 entities
• PEXT_ENTITYDBL2 - Up to 2048 entities
• PEXT_MODELDBL - Up to uint16 model indices
  • svc_fte_modellistshort
• PEXT_TRANS - 1 byte alpha field on entities
• PEXT_COLOURMOD - 3 byte colourmod on entities

Watching a stream of with alpha will require an ezQuake version newer than 2013, and if colormod is used, clients need to be 3.6.6 or newer. For alpha brushes to actually render correctly ezQuake 3.6.6 or newer needs to be used.

[dsvensson]

Just need to reboot and see if I have any more changes there.

Verified that old client can qtvplay this correctly if a map lacks alpha entities and colourmod is not in use, and if no colormod, outdated clients will not drop connection, just render stuff opaque. It correctly forwards data for both plain old mvdsv as well as new mvdsv+alpha/colormod, and it correctly streams both variants to the hub, where old mvdsv ofc lacks the alpha. Just have some final checks of various permutations during the day and this can be merged.