Windows QPA/GL: Avoid crash in GL driver.

QWindowsStaticOpenGLContext::doCreate(): Invoke
QWindowsOpenGLTester::supportedRenderers() only when needed; specifically,
do not unnecessarily call it when GLES/SW renderer is specified.
Amends change b4c8e151

.

Task-number: QTBUG-49541
Task-number: QTBUG-52693
Change-Id: I58d1c584d194f8e7fee35ee16b77575f86d3c22e
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>