DarkXL crashes on shutdown with software renderers

[psi29a]

Happens with soft8, but will likely happen with soft32 when #18 is fixed.

*** Error in `/home/bcurtis/Workspace/Private/XLEngine/build/XLEngine': free(): invalid pointer: 0x000055555a2b2ce0 ***

Thread 1 "XLEngine" received signal SIGABRT, Aborted.
__GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:51
51	../sysdeps/unix/sysv/linux/raise.c: No such file or directory.
(gdb) bt
#0  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:51
#1  0x00007ffff5fdef5d in __GI_abort () at abort.c:90
#2  0x00007ffff602728d in __libc_message (action=action@entry=do_abort, 
    fmt=fmt@entry=0x7ffff614e528 "*** Error in `%s': %s: 0x%s ***\n") at ../sysdeps/posix/libc_fatal.c:181
#3  0x00007ffff602e64a in malloc_printerr (action=<optimized out>, str=0x7ffff614aeae "free(): invalid pointer", 
    ptr=<optimized out>, ar_ptr=<optimized out>) at malloc.c:5426
#4  0x00007ffff603073e in _int_free (av=0x7ffff6380c20 <main_arena>, p=<optimized out>, have_lock=0) at malloc.c:4175
#5  0x00007ffff603544e in __GI___libc_free (mem=<optimized out>) at malloc.c:3145
#6  0x00005555556cc1d1 in aligned_free (ptr=0x55555a2b2ce0)
    at /home/bcurtis/Workspace/Private/XLEngine/xlengine/render/Driver3D_Soft.cpp:84
#7  0x00005555556cc4b6 in Driver3D_Soft::~Driver3D_Soft (this=0x555557f9ad40, __in_chrg=<optimized out>)
    at /home/bcurtis/Workspace/Private/XLEngine/xlengine/render/Driver3D_Soft.cpp:129
#8  0x00005555556cc584 in Driver3D_Soft::~Driver3D_Soft (this=0x555557f9ad40, __in_chrg=<optimized out>)
    at /home/bcurtis/Workspace/Private/XLEngine/xlengine/render/Driver3D_Soft.cpp:134
#9  0x00005555557366e0 in Engine::Destroy (this=0x555557fffa20) at /home/bcurtis/Workspace/Private/XLEngine/xlengine/Engine.cpp:369
#10 0x00005555557356fe in Engine::~Engine (this=0x555557fffa20, __in_chrg=<optimized out>)
    at /home/bcurtis/Workspace/Private/XLEngine/xlengine/Engine.cpp:96
#11 0x000055555573571a in Engine::~Engine (this=0x555557fffa20, __in_chrg=<optimized out>)
    at /home/bcurtis/Workspace/Private/XLEngine/xlengine/Engine.cpp:97
#12 0x0000555555799b36 in main (argc=3, argv=0x7fffffffdf18)
    at /home/bcurtis/Workspace/Private/XLEngine/xlengine/linux/main.cpp:236