[Crash] Using ShaderEffect can cause crashes or hangs

http://codereview.qt-project.org/#change,14800 has unearthed various problems with ShaderEffect, including crashing on QQuickView dtor, and spinning when syncing to scene graph (backtrace attached).

(gdb) r
^C
Program received signal SIGINT, Interrupt.
0xb742d30a in QSGNode::markDirty (this=0xb5d03500, bits=...) at scenegraph/coreapi/qsgnode.cpp:481
481 while (p) {
(gdb) bt
#0 0xb742d30a in QSGNode::markDirty (this=0xb5d03500, bits=...) at scenegraph/coreapi/qsgnode.cpp:481
#1 0xb742cdb6 in QSGNode::removeChildNode (this=0xb5d03500, node=0xb5d03500) at scenegraph/coreapi/qsgnode.cpp:367
#2 0xb749b3ea in QQuickCanvasPrivate::updateDirtyNode (this=0x80d36f8, item=0x82c6d78) at items/qquickcanvas.cpp:1517
#3 0xb749aece in QQuickCanvasPrivate::updateDirtyNodes (this=0x80d36f8) at items/qquickcanvas.cpp:1456
#4 0xb7494e95 in QQuickCanvasPrivate::syncSceneGraph (this=0x80d36f8) at items/qquickcanvas.cpp:228
#5 0xb759c130 in QQuickTrivialWindowManager::renderCanvas (this=0x80c7dd8, canvas=0x80aa2b0) at items/qquickwindowmanager.cpp:1160
#6 0xb759c545 in QQuickTrivialWindowManager::event (this=0x80c7dd8, e=0x82cb768) at items/qquickwindowmanager.cpp:1245
#7 0xb78f8c02 in QApplicationPrivate::notify_helper (this=0x807cdc8, receiver=0x80c7dd8, e=0x82cb768) at kernel/qapplication.cpp:4045
#8 0xb78f6487 in QApplication::notify (this=0xbffff5b8, receiver=0x80c7dd8, e=0x82cb768) at kernel/qapplication.cpp:3462
#9 0xb61242cd in QCoreApplication::notifyInternal (this=0xbffff5b8, receiver=0x80c7dd8, event=0x82cb768) at kernel/qcoreapplication.cpp:676
#10 0xb6127b93 in QCoreApplication::sendEvent (receiver=0x80c7dd8, event=0x82cb768) at kernel/qcoreapplication.h:205
#11 0xb61252ca in QCoreApplicationPrivate::sendPostedEvents (receiver=0x0, event_type=0, data=0x807ce98) at kernel/qcoreapplication.cpp:1312
#12 0xb6124db9 in QCoreApplication::sendPostedEvents (receiver=0x0, event_type=0) at kernel/qcoreapplication.cpp:1173
#13 0xb617f20e in QCoreApplication::sendPostedEvents () at kernel/qcoreapplication.h:210
#14 0xb617da92 in postEventSourceDispatch (s=0x808b910) at kernel/qeventdispatcher_glib.cpp:279
#15 0xb5af45e5 in g_main_context_dispatch () from /lib/libglib-2.0.so.0
#16 0xb5af82d8 in ?? () from /lib/libglib-2.0.so.0
#17 0xb5af84b8 in g_main_context_iteration () from /lib/libglib-2.0.so.0
#18 0xb617eac6 in QEventDispatcherGlib::processEvents (this=0x8086220, flags=...) at kernel/qeventdispatcher_glib.cpp:424
#19 0xb612471a in QCoreApplication::processEvents (flags=..., maxtime=50) at kernel/qcoreapplication.cpp:873
#20 0x0804ba98 in QTest::qWait (ms=50) at ../../../../../qtbase/include/QtTest/../../src/testlib/qtestsystem.h:69
#21 0x0804f942 in tst_qquickshadereffect::deleteShaderEffectSource (this=0xbffff5b0) at tst_qquickshadereffect.cpp:304