Use std::vector range ctor in QVector::toStdVector()

There are three reasons to do so:

1. This could be more efficient, depending on the STL implementation.

2. By using QTypedArrayData iterators (T*) instead of QVector ones,
   we actually invoke the non-templated range ctor of std::vector,
   at least in the common case that std::vector<T>::const_iterator
   is also const T*.

3. The change turns a former NRVO return into a RVO one, potentially
   allowing more compilers to perform the copy elision.

Change-Id: I70b35aaeae70ba06a971a36b8b1b1da997e8094f
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>