Calling setView or update on NSOpenGLContext results in recreating the
internal GL surfaces of the view. Unfortunately there seems to be a
fixed amount of these surfaces available, so if we spin a loop where
we for some reason end up recreating them, we'll easily run out, and
lock up the whole window system:

  thread #6, name = 'SwapThread'
    frame #0: 0x00007fff7b45220a libsystem_kernel.dylib`mach_msg_trap + 10
    frame #1: 0x00007fff7b451724 libsystem_kernel.dylib`mach_msg + 60
    frame #2: 0x00007fff751c1675 SkyLight`SLSBindSurface + 247
    frame #3: 0x00007fff5d9c4328 OpenGL`___lldb_unnamed_symbol29$$OpenGL + 255
    frame #4: 0x00007fff6bf42c33 libGPUSupportMercury.dylib`gldAttachDrawable + 364
    frame #5: 0x00007fff5d9e61e7 GLEngine`gliAttachDrawableWithOptions + 257
    frame #6: 0x00007fff5d9c4bb0 OpenGL`___lldb_unnamed_symbol38$$OpenGL + 969
    frame #7: 0x00007fff5d9c8b0e OpenGL`___lldb_unnamed_symbol57$$OpenGL + 82
    frame #8: 0x00007fff5d9c8e55 OpenGL`CGLSetSurface + 330
    frame #9: 0x00007fff50d0eb2c AppKit`NSOpenGLContextAttachOffScreenViewSurface + 352

This can happen e.g. when resizing the application, where AppKit itself spins
a loop where we don't end up back in QCocoaEventDispatcher::processEvents()
for each pass (where we do have a local pool). Or it can happen in the
render-loop of a render-thread that doesn't use the event dispatcher.

Change-Id: Iaf2f879dd01e3d807d0f35705ccc978dbc89036b
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>