Move CopyMetadata to Init in MP4FileStream

Fixes a crash when closing the MP4FileStream before actually
writing/receiving any packets
Fix typo in Replay Buffer
Disable FastAlloc for 64 bit builds

FastAlloc only uses the lower 32 bits of a pointer to index its internal
structures which is a problem for allocations with size >= 0x8001,
because multiple big allocations can have the same values for bit 16 to
bit 32, which causes FastAlloc to discard its info about the previous
allocation
Release packets earlier in the ReplayBuffer

With very high bitrate settings the replay buffer would severely violate
the memory usage estimate because it wouldn't release any frames until
it was done writing the file, which could take a long time depending
on the storage subsystem
Workaround github API issue
Use absolute paths for opening files where possible

Works around various 3rd party nasties that change the working directory and don't set it back properly.