Invalidate font caches when switching between threads

The font caches can only be used from a single thread at a time.
QFontEngineFT for instance, uses a global static thread storage
which is accessed on releasing and creating engines, and this
causes a crash if the font engine is created on one thread and
released on another.

We use the updatePolish() function to make sure the caches are
empty before entering updatePaintNode(), and then we invalidate
the cache again after updatePaintNode() is done.

[ChangeLog][Text] Fixed uncommon crash in text nodes.

Change-Id: I01dbc2ed58aeebd03d77a157c700330334bdb385
Task-number: QTBUG-38800
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>