Add a new optimization property to QGraphicsScene

The minimumRenderSize is a qreal value that is used as a lower bound to
determine what items are visible when a scene is rendered. If an item's
view-transformed width or height are less than minimumRenderSize then
this item is considered to insignificantly affect the final result and
is not drawn. If the item clips its children to its shape they are
automatically not drawn. This greatly reduces the drawing overhead
for scenes with many items rendered in a zoomed out view.

[ChangeLog][QtWidgets][QGraphicsScene] Added the minimumRenderSize
property which can be used to speed up rendering by not painting
items, smaller than a give size.

Change-Id: Ie208234707dffb4d2fc620fc5d1514e0c144d9a8
Reviewed-by: Andreas Aardal Hanssen <andreas@hanssen.name>