The resize event should not be used to draw, as there will be a follow-up
exposeEvent delivered just after, and we don't want to draw twice.
This is how QRasterWindow operates too, except it defers the backingstore
resize to beginPaint() instead of resizing in the reizeEvent.
It's also how QOpenGLWindow operates, which also has a note for the
virtual resizeGL method saying: "Scheduling updates from here is not
necessary. The windowing systems will send expose events that trigger
an update automatically."
Change-Id: I2a9740018508c2eb129149f53237ee8e378c03b1
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>
9c17b8adb9