QNX: Ensure that m_requestedBufferSize is properly initialised

QNX: Ensure that m_requestedBufferSize is properly initialised

This solves a new corner case resulting from the recent refactoring of
QQnxWindow::setGeometry() that occurs when using a plain OpenGL backed
window in a single threaded environment and the user code calls
QOpenGLContext::makeCurrent(this) in the QWindow subclass ctor.

Change-Id: Iaf07fbff1cb2743f5706020a8d657a82cb9f2255
Reviewed-by: Kevin Ottens <kevin.ottens.qnx@kdab.com>
Reviewed-by: Kevin Krammer <kevin.krammer@kdab.com>
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>