rhi: gl: Set the screen on the context, if possible

Take the screen from the window, if the window was specified.

When it comes to Qt Quick, this is in fact required in order to
make the behavior on par with Qt 5, see
https://code.qt.io/cgit/qt/qtdeclarative.git/tree/src/quick/scenegraph/qsgthreadedrenderloop.cpp?h=5.15#n1336
try to mirror that in Qt 6 as well. There are still subtle differences
between Qt 5 and Qt 6 in particular when qt_gl_global_share_context is
set (do we set the screen from that or from the window?), for now leave
that question unsettled.

What exactly setting the screen on the context does is platform
specific, and can become relevant with multi screen, multi GPU
systems.

Pick-to: 6.2
Change-Id: Icc90b8fea87bf1e34ecf1dec0702f4d3c411db00
Reviewed-by: Christian Strømme <christian.stromme@qt.io>

src/gui/rhi/qrhigles2.cpp

@@ -484,6 +484,8 @@ bool QRhiGles2::create(QRhi::Flags flags)
         } else if (QOpenGLContext *shareContext = qt_gl_global_share_context()) {
             ctx->setShareContext(shareContext);
             ctx->setScreen(shareContext->screen());
+        } else if (maybeWindow) {
+            ctx->setScreen(maybeWindow->screen());
         }
         if (!ctx->create()) {
             qWarning("QRhiGles2: Failed to create context");