[plugins/qpa] Don't crash on Qt5.12 OpengGL::makeCurrent

Summary:
QOpenGLContext was changed to call platform->makeCurrent before it
called setCurrentContext.

Because we bind window FBO and ultimately that calls
QOpenGlContext::format we need Qt to know which context is current so it
can tell us the format.

This matches the QtWayland EGL code.

BUG: 399392

Test Plan:
Switched virtual desktop with the OSD (previously reliably crashed)
Now it doesn't

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D15957

plugins/qpa/sharingplatformcontext.cpp

@@ -26,6 +26,7 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #include <logging.h>
 
 #include <QOpenGLFramebufferObject>
+#include <private/qopenglcontext_p.h>
 
 namespace KWin
 {
@@ -48,6 +49,8 @@ SharingPlatformContext::SharingPlatformContext(QOpenGLContext *context, const EG
 bool SharingPlatformContext::makeCurrent(QPlatformSurface *surface)
 {
     Window *window = static_cast<Window*>(surface);
+
+    QOpenGLContextPrivate::setCurrentContext(context());
     if (eglMakeCurrent(eglDisplay(), m_surface, m_surface, eglContext())) {
         window->bindContentFBO();
         return true;