PVS: dynamically allocate all FatPVS buffers

PVS: dynamically allocate all FatPVS buffers

Some were dynamic, others weren't...

Fixes invisible entities and monsters ignoring the player on big/complex
maps, and undefined behaviour (reading past the end of the buffer).
Fixes https://gitlab.com/xonotic/darkplaces/-/issues/322

Adds Mem_Size() for retrieving the size of an allocation.

Simplifies some checks (we can check the pointer now so we dont need to
store byte counts just to see if there's any data).

Signed-off-by: bones_was_here <bones_was_here@xonotic.au>