Thanks for looking at the bug.  Following your instructions, got this: 

jerry@jerry-AOD255E:~$ sudo gdb /usr/bin/compiz /tmp/unity-crash/CoreDump
GNU gdb (Ubuntu 7.8-1ubuntu4) 7.8.0.20141001-cvs
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Reading symbols from /usr/bin/compiz...(no debugging symbols found)...done.
[New LWP 2018]
[New LWP 2023]
[New LWP 2045]
[New LWP 2534]
[New LWP 2046]
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
Core was generated by `compiz'.
Program terminated with signal SIGSEGV, Segmentation fault.
#0  0x00007f4ebcc67439 in unity::UnityScreen::paintDisplay (this=this@entry=0xb0f000)
    at /build/buildd/unity-7.3.1+14.10.20141016/plugins/unityshell/src/unityshell.cpp:837
837	/build/buildd/unity-7.3.1+14.10.20141016/plugins/unityshell/src/unityshell.cpp: No such file or directory.

(gdb) bt full
#0  0x00007f4ebcc67439 in unity::UnityScreen::paintDisplay (this=this@entry=0xb0f000)
    at /build/buildd/unity-7.3.1+14.10.20141016/plugins/unityshell/src/unityshell.cpp:837
        blur_region = {priv = 0x1cdee60}
        gpu_device = <optimized out>
        backup_texture_id = <optimized out>
        surface_target = <optimized out>
        output = 0x65fa20
        current_draw_binding = 1
        old_read_binding = 1
        outputGeo = <optimized out>
#1  0x00007f4ebcc6779a in unity::UnityScreen::glPaintOutput (this=0xb0f000, attrib=..., transform=..., region=..., output=0x65fa20, mask=3)
    at /build/buildd/unity-7.3.1+14.10.20141016/plugins/unityshell/src/unityshell.cpp:1421
        ret = true
        force = false
#2  0x00007f4ec7bdc7c8 in GLScreen::glPaintOutput(GLScreenPaintAttrib const&, GLMatrix const&, CompRegion const&, CompOutput*, unsigned int)
    () from /usr/lib/compiz/libopengl.so
No symbol table info available.
#3  0x00007f4ec7be4415 in PrivateGLScreen::paintOutputs(std::list<CompOutput*, std::allocator<CompOutput*> >&, unsigned int, CompRegion const&) () from /usr/lib/compiz/libopengl.so
No symbol table info available.
#4  0x00007f4ed42f81d6 in CompositeScreen::paint(std::list<CompOutput*, std::allocator<CompOutput*> >&, unsigned int) ()
   from /usr/lib/compiz/libcomposite.so
---Type <return> to continue, or q <return> to quit---
No symbol table info available.
#5  0x00007f4ed42fb92c in CompositeScreen::handlePaintTimeout() () from /usr/lib/compiz/libcomposite.so
No symbol table info available.
#6  0x00007f4edc5a31ed in CompTimer::triggerCallback() () from /usr/lib/libcompiz_core.so.ABI-20140123
No symbol table info available.
#7  0x00007f4edc5a32bf in CompTimeoutSource::callback() () from /usr/lib/libcompiz_core.so.ABI-20140123
No symbol table info available.
#8  0x00007f4edc5a276d in CompTimeoutSource::dispatch(sigc::slot_base*) () from /usr/lib/libcompiz_core.so.ABI-20140123
No symbol table info available.
#9  0x00007f4edaa7e10f in Glib::Source::dispatch_vfunc(_GSource*, int (*)(void*), void*) () from /usr/lib/x86_64-linux-gnu/libglibmm-2.4.so.1
No symbol table info available.
#10 0x00007f4eda76db6d in g_main_context_dispatch () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
No symbol table info available.
#11 0x00007f4eda76df48 in ?? () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
No symbol table info available.
#12 0x00007f4eda76e272 in g_main_loop_run () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
No symbol table info available.
#13 0x00007f4edc55c96b in compiz::private_screen::EventManager::startEventLoop(_XDisplay*) () from /usr/lib/libcompiz_core.so.ABI-20140123
No symbol table info available.
#14 0x0000000000401931 in main ()
No symbol table info available.
(gdb) q
jerry@jerry-AOD255E:~$ 

Anything else?

Thanks, Jerry