Disable Vec3 pooling for thread safety. Fixes #240.

Some mods access Vec3Pool in a thread-unsafe manner, causing
java.lang.NullPointerException at aok.a(Vec3Pool.java:123) crashes
on MCPC+, fairly frequently in some configurations. Remove the
pooling of Vec3 objects and instead always return a new object;
should be acceptable on modern JVMs.

Thanks to nallar for this fix.