Merge signon buffer fragmentation patch from Quakespasm / Ironwail

[vsonnier]

• Try to merge https://github.com/sezero/quakespasm/commit/98baeeffb1e7e24ed408c1e341fde7f74760d602 then https://github.com/sezero/quakespasm/commit/b278c986831d8125b478ef6d75f35828e6af4804 (originally from Ironwail)
• Take care of keeping existing demo seeking code working
• No way to test it multiplayer anyway

[vsonnier]

Closing this, a copy has been made on the original repo here : https://github.com/Novum/vkQuake/issues/701