Update fbo toImage() docs regarding premultiplied alpha

Task-number: QTBUG-42510
Task-number: QTBUG-37261
Change-Id: Ic11bec0a25e66df9d022f640621686be867e84d2
Reviewed-by: Gunnar Sletta <gunnar@sletta.org>
QPanGestureRecognizer: Make the number of touch points a parameter.

Prepare for determining the suitable number of touch points
from the device type. For now, 2 points are used as
before, which can be overridden by setting the environment
variable QT_PAN_TOUCHPOINTS. Add member variable
to QPanGesturePrivate which is set on gesture creation and later
used for comparison.

Task-number: QTBUG-40461
Change-Id: I6d9e35ca752375bc6a54435482ca0925195b8142
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>
Fix broken resource cleanup in QOpenGLWidget

The context has to be made current. Otherwise we may fail to clean up or,
what's worse, we may delete FBOs and textures in some random context.

The latter was visible with ANGLE in the qopenglwidget example. When having
two QOpenGLWidget instances, the context for the second happened to be the
current one when destroying the first. This is now avoided by making sure the
correct context is current when deleting the FBOs.

Task-number: QTBUG-42696
Change-Id: I8c1eed7c13a869968cc67141e585d02c6bc6f279
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>
Fix missing docs for QByteArrayList

The public is needed for qdoc which sees class, not struct.

Task-number: QTBUG-42689
Change-Id: I28298b5fd13c6841838634a440bb2f726ddbe7be
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>