As far as the set of .cpp files which are using vecmat.h are
concerned, `Zero_vector` is out of reach for the compiler optimizer,
because it is extern / lives in a separate translation unit. An
expression like `x == Zero_vector` or `v = Zero_vector` thus has to
perform memory loads (for Zero_vector's x,y,z parts) before
comparison or copying, respectively. By using an immediate zero
vector `vector{}` instead, the unnecessary extra loads should go
away.
I present exhibit A:
```
void copyxx(vector *x) { *x = Zero_vector; }
4905e0: 48 8b 05 41 c0 56 00 movq 0x56c041(%rip),%rax # 9fc628 <Zero_vector>
4905e7: 48 89 07 movq %rax,(%rdi)
4905ea: 8b 05 40 c0 56 00 movl 0x56c040(%rip),%eax # 9fc630 <Zero_vector+0x8>
4905f0: 89 47 08 movl %eax,0x8(%rdi)
4905f3: c3 ret
```
vs.
```
void copyxx(vector *x) { *x = vector{}; }
4905c0: 48 c7 07 00 00 00 00 movq $0x0,(%rdi)
4905c7: c7 47 08 00 00 00 00 movl $0x0,0x8(%rdi)
4905ce: c3 ret
```
Possibly fixes#530.
physics/findintersection.cpp:4711:64: runtime error: index -1 out of bounds for type 'fvi_face_room_list [200]'
=================================================================
SIGNAL 11 caught, aborting
As level 1's in-engine cutscene runs, an OOB happens after about 3s.
x=0 object[x].type=4
x=1 object[x].type=18
x=2 object[x].type=255
$GIT/physics/findintersection.cpp:2196:80: runtime error: index 255 out of bounds for type 'unsigned char [26]'
$GIT/physics/findintersection.cpp:2196:80: runtime error: load of address 0x00000475a97f with insufficient space for an object of type 'uint8_t'
==28871==ERROR: AddressSanitizer: global-buffer-overflow on address 0x00000475a97f at pc 0x000001242275 bp 0x7fffc1ced720 sp 0x7fffc1ced718
READ of size 1 at 0x00000475a97f thread T0
f0 fvi_QuickDistObjectList(vector*, int, float, short*, int, bool, bool, bool, bool) $GIT/physics/findintersection.cpp:2196
f1 AIDoMemFrame $GIT/Descent3/AImain.cpp:6066
f2 AIDoFrame(object*) $GIT/Descent3/AImain.cpp:6187
f3 ObjDoFrame(object*) $GIT/Descent3/object.cpp:2674
f4 ObjDoFrameAll() $GIT/Descent3/object.cpp:2988
f5 GameFrame() $GIT/Descent3/GameLoop.cpp:2980
f6 GameSequencer() $GIT/Descent3/gamesequence.cpp:1221
f7 PlayGame() $GIT/Descent3/game.cpp:834
f8 MainLoop() $GIT/Descent3/descent.cpp:555
f9 Descent3() $GIT/Descent3/descent.cpp:508
f10 oeD3LnxApp::run() $GIT/Descent3/sdlmain.cpp:151
0x00000475a97f is located 30 bytes after global variable 'FVI_always_check_ceiling' defined in '$GIT/physics/findintersection.cpp:888:6' (0x475a960) of size 1
'FVI_always_check_ceiling' is ascii string ''
0x00000475a97f is located 33 bytes before global variable 'fvi_visit_list' defined in '$GIT/physics/findintersection.cpp:895:5' (0x475a9a0) of size 51
