fix crashes in QGIS 3D, mostly directly at project load

[PeterPetrik]

Fix the random crashes, usually on LOADING of 3d view. The stacktrace of those errors could be mostly ANYTHING :), and sometimes some random stuff like

0   org.qt-project.QtGui           0x000000010be75af9 qt_memfillXX_avx2(unsigned char*, long long vector[4], long long) + 41
1   org.qt-project.QtGui           0x000000010bef1bd5 QImage::fill(Qt::GlobalColor) + 37
2   org.qgis.qgis3_core            0x000000010ebf2740 QgsMapRendererSequentialJob::QgsMapRendererSequentialJob(QgsMapSettings const&) + 320

or

1   org.qt-project.Qt3DCore        0x00000001109ecde7 Qt3DCore::QScene::removePropertyTrackDataForNode(Qt3DCore::QNodeId) + 71
2   org.qt-project.Qt3DCore        0x00000001109fe708 Qt3DCore::QNodePrivate::setScene(Qt3DCore::QScene*) + 40
3   org.qt-project.Qt3DCore        0x00000001109fd957 Qt3DCore::QNodePrivate::unsetSceneHelper(Qt3DCore::QNode*) + 199
4   org.qt-project.Qt3DCore        0x0000000110a00bd1 void 

or

6   libsystem_malloc.dylib         0x00007fff73523cf5 malloc + 21
7   org.qt-project.QtCore          0x000000010ab40bc0 QArrayData::allocate(unsigned long, unsigned long, unsigned long, QFlags<QArrayData::AllocationOption>) + 144
8   org.qt-project.QtCore          0x000000010ab9cea3 QByteArray::QByteArray(char const*, int) + 51
9   org.qt-project.Qt3DRender      0x000000011534a210 Qt3DRender::QTextureImageData::setImage(QImage const&) + 128
10  org.qgis.qgis3_3d              0x000000010c6bd3c4 TerrainTextureImageDataGenerator::operator()() + 148
11  org.qt-project.Qt3DRender      0x0000000115388bbe Qt3DRender::Render::GLTexture::loadTextureDataFromImages() + 94

or

0   libsystem_kernel.dylib         0x00007fff734aedfa mach_msg_trap + 10
1   libsystem_kernel.dylib         0x00007fff734af170 mach_msg + 60
2   com.apple.AppKit               0x00007fff36ae9fe6 NSWaitUntilHostTime + 102
3   com.apple.AppKit               0x00007fff36f92ef0 -[NSCGLSurface synchronize] + 105
4   com.apple.AppKit               0x00007fff36f923c7 NSCGLSurfaceFlush + 89
5   GLEngine                       0x00007fff43e2992a glSwap_Exec + 186
6   com.apple.opengl               0x00007fff43e0b473 CGLFlushDrawable + 59
7   com.apple.AppKit               0x00007fff3691875a -[NSOpenGLContext flushBuffer] + 20
8   libqcocoa.dylib                0x00000001181745fd QCocoaGLContext::swapBuffers(QPlatformSurface*) + 509
9   org.qt-project.QtGui           0x0000000108f0508c QOpenGLContext::swapBuffers(QSurface*) + 

Due to the fact, that in this multi-threaded rendering, the memory is kind-of-corrupted (two processes are trying to persist the new data block to the same resulting member array variable)

fixes #35483