Vega Strike Code

Fix segfault when building an iterator for a collection of killed units
The iterator was being registered too late (after calling erase) so
erase was performing unsafe list manipulations.

May fix #680 and related errors.

Telltale signs of this bug are crashes shortly after a bunch of subunits
get killed (UNIT HAS DIED in the last few lines of log output), which
happen more often than not when killing units with missiles.

Optimize condition for empty collections to avoid computing the size of
a linked list, which is O(N), whenever possible

Remove useless argument that's just the default value

Small profile-guided optimization: simplify GetWarpVelocity when not in warp

Small profile-guided optimization: Unit::nummesh() doesn't really need
to be virtual, and having it so is burning lots of cycles in hot
draw/physics loops, so until it becomes necessary, make it nonvirtual
and inline

Small profile-guided optimization: avoid handling the heap if the new
occluder is going to be excluded anyway (ie: less relevant than the
least relevant occluder in a full heap)

Small profile-guided optimizations:
- replace repeated division with multiplication by reciprocal
- make matrix fillup sequential in memory order

Fix optimized FX update loop to not be infinite

Small profile-guided optimization: update effects backwards to minimize
redundant data movement when removing lights

Small profile-guided optimization: avoid unnecessary work in the physics loop