This allows anticipating and reusing internal allocations of
QPainterPathElements instead of using the common `m_myPath = QPainterPath{}` pattern.
[ChangeLog][QtGui][QPainterPath] Added clear(), reserve(), capacity().
clear() removes allocated QPainterPath elements but preserves allocated memory, which can be
useful for application with complex paths that are often recreated. reserve() and capacity()
follow QVector semantics.
Change-Id: I763461e2a421feda9053d3eb512af2fcf07ade2b
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>
e6a7b61d27