Unify all mid() functions in QtBase.

Up to now, Qt had at least 3 different implementations of the mid().
Only QString::mid implementation was not crashing on edge cases and
was protected against overflows, therefore I picked that one as the
base implementation, even if it has weird semantics for an invalid
input.

As a side effect QVector::mid was slightly optimized to not detach in
all cases (which follows current QList behavior). Documentation of
QVector::mid and QList::mid was updated to not mention "copy of data"
which could suggest that the mid() result is detached.

QStringRef::mid was fixed and now it follows general Qt behavior, by
returning a null value for a null input.

Change-Id: Ie9ff5d98372bd193d66508e6dd92b6ed1180ad9b
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Add viewport transform to QMatrix4x4

This allows to easily create a matrix that performs the transformation
used by OpenGL fixed function to go from normalized device coordinates
to window coordinates.

This comes in useful if you need to perform the NDC->window coordinate
conversion inside a shader.

Change-Id: I183b3545bfb3eb1e8b13fc3172911b46926fcbb7
Reviewed-by: Paul Lemire <paul.lemire@kdab.com>
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>
Expose multisample settings on QOpenGLTexture

Change-Id: I877f4139aed8bb03b798818a3fac00dab1523ce1
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>