widgets: Mark widgets as needing flush during painting

Except for the case of syncing in response to an expose event,
where the platform asked us to flush a specific region, we
should strive to only flush parts that have been repainted.

And we should flush those parts to their nearest native child,
instead of unconditionally flushing the root/top level widget
as well.

By allowing drawWidget to schedule the flush we automatically
flush the minimal region, to the right widgets.

(manually cherry-picked from commit 79bf1b7e348d186934b14c417859a48bf9b3a06c)

Task-number: QTBUG-77447
Change-Id: I1a8fa703c0b7172655271e2a8f5eca694cb14129
Reviewed-by: Paul Olav Tvete <paul.tvete@qt.io>

0 files changed, 0 insertions, 0 deletions