Clear a stray ref on layers when purging GL resources

Fix a regression of 3df57f7e550a19cb4450ed3896aa51ddb38c2ed2 where
a reference to TextureMapperBackingStore would be kept in the
TextureMapperLayers after purgeGLResources(). This would crash later
on if the window would get destroyed before the
CoordinatedGraphicsScene that own those layers.

Task-number: QTBUG-40734
Change-Id: Id1a596c0f0ba9afd1e163f83c38dda939f521314
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@theqtcompany.com>