Code review comment for lp:~gerboland/miral/add-interactivity-to-WM

Crash seems fairly consistent. If it matters, this is g++/yakkety:

Program terminated with signal SIGSEGV, Segmentation fault.
#0 0x00007fdbe02508fb in qtmir::MirSurface::updateTexture (this=0x55d46b5f13a0)
    at /home/alan/display_server/miral0/miral-qt/src/modules/Unity/Application/mirsurface.cpp:406
406 auto renderables = window->generate_renderables(userId);
[Current thread is 1 (Thread 0x7fdbce4d5700 (LWP 9694))]
(gdb) bt
#0 0x00007fdbe02508fb in qtmir::MirSurface::updateTexture (this=0x55d46b5f13a0)
    at /home/alan/display_server/miral0/miral-qt/src/modules/Unity/Application/mirsurface.cpp:406
#1 0x00007fdbe025bf81 in qtmir::MirSurfaceItem::updatePaintNode (
    this=0x55d46b418ac0, oldNode=0x7fdbc4569bd0)
    at /home/alan/display_server/miral0/miral-qt/src/modules/Unity/Application/mirsurfaceitem.cpp:226
#2 0x00007fdc011132c8 in QQuickWindowPrivate::updateDirtyNode(QQuickItem*) ()
   from /usr/lib/x86_64-linux-gnu/libQt5Quick.so.5
#3 0x00007fdc01113b3b in QQuickWindowPrivate::updateDirtyNodes() ()
   from /usr/lib/x86_64-linux-gnu/libQt5Quick.so.5
#4 0x00007fdc01113cb0 in QQuickWindowPrivate::syncSceneGraph() ()
   from /usr/lib/x86_64-linux-gnu/libQt5Quick.so.5
#5 0x00007fdc010e4d53 in ?? () from /usr/lib/x86_64-linux-gnu/libQt5Quick.so.5
#6 0x00007fdc010e5f95 in ?? () from /usr/lib/x86_64-linux-gnu/libQt5Quick.so.5
#7 0x00007fdc010eb07c in ?? () from /usr/lib/x86_64-linux-gnu/libQt5Quick.so.5
#8 0x00007fdc001d77be in ?? () from /usr/lib/x86_64-linux-gnu/libQt5Core.so.5
#9 0x00007fdbff2ab70a in start_thread (arg=0x7fdbce4d5700) at pthread_create.c:333
#10 0x00007fdbff8d06ad in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:109