Introduce pool allocator for scheduled efx

Before this commit, efx were scheduled to be played by dynamically
allocating memory every time, and then freed when the efx's life time had
ended. Introducing a pool allocator potentially removes the overhead of
many dynamic allocations per frame.

Further work needs to be done to remove dynamic allocations for the
effects themselves (see FX_AddLine, FX_AddBezier, etc. functions).
Merge pull request #506 from mrwonko/jk2-clean

JK2 support in separate executable/library; option to compile renderers
Merge branch 'master' of github.com:Razish/OpenJK
[SP] JK2: Don't build JK2 stuff by default since its not in working order for packaging and such, and unnecessary projects that people would just turn off all the time if going for JA support.